By aspect — in detail
The whole pitch is a tunable Harman: a warm-leaning, bass-lifted take on the Harman target that you can reshape with the switches. Reviewers agree the switches 'mainly change the bass level' (with a smaller mid-treble lift), so one set covers neutral, mild-V and bass-forward — flexibility that's rare at the price. The default of this Improved-Bass version reads warm and full rather than strictly neutral.
“The KZ Castor Pro is a refined, slightly elevated version of the original Castor.”
geumja-ssi, r/iems
“You get the freedom to switch between different sound signatures such as neutral, V-shaped, bass-heavy, or relaxed”
The Audio Store
“they mainly change the bass level, with some small changes in other frequencies that may or may not be noticeable”
dr_wtf, r/iems
“Highly tunable sound with effective switches”
RedditRecs (aggregated from 274 Reddit reviews)
Measured
A Harman-leaning response with an elevated low end: a sub-bass shelf, a mild lower-midrange dip, a pinna-gain rise around 2.5–3 kHz and upper-treble energy near 8–10 kHz (The Audio Store's graph; the Castor_Pro_Imp_Bass curves on squig.link). KZ's switch scheme lifts the bass one or two levels (switches 1, 1+2) and the mid-highs a level (switch 3). On the original Castor, ASR found switches 1 and 2 brought it closest to Harman while turning everything on overshot the treble.
Sub-bass-forward with a dedicated low-frequency driver, and in its bassier switch modes (UUDD/UUUU) most hear it as powerful, punchy and deep — one of the set's headline strengths and a top-rated trait in the community aggregate. But the 'improved bass' name cuts both ways: a real minority find it surprisingly light, and that split tracks physical causes — a poor tip seal, or leaving the bass switches down. Some also note the plain Harman version's bass is 'totally original, not boosted,' which fuels the confusion between the two SKUs.
Measured
The Improved-Bass version, per KZ, 'boosts both sensitivity and dynamic performance in the low-frequency range' over the straight-Harman Castor Pro, and the switches add roughly a level or two of low-end. Because a dynamic driver's bass leans on a tight seal, perceived quantity swings a lot with tips and fit — the same reason ASR had to physically push the tips in for full bass on the original Castor.
Where it splits
Surprisingly light — even on the Enhanced/Improved Bass edition, some don't get much bass.35%
“Even after it being Enhanced Bass Edition, I don't find much bass.”
kabir-doha, r/iemlndia
Clear and clean, with a deliberate mild dip in the lower midrange (~300 Hz–1 kHz) that keeps things from congesting and keeps vocals from turning shouty. Most hear good vocal clarity, especially on male voices; the trade-off, flagged in the community aggregate, is that the mids can read as slightly recessed or lacking warmth behind the elevated bass.
“The midrange remains clear and detailed, with good vocal clarity, especially for male vocals.”
geumja-ssi, r/iems
“Instruments are placed well and never feel shouty or overly emphasized.”
The Audio Store
“Recessed or unclear mids/vocals”
RedditRecs (aggregated from 274 Reddit reviews)
Measured
A small lower-midrange dip around 300 Hz–1 kHz (The Audio Store) sits under the pinna-gain rise, which is why the mids read clean and uncongested but a touch recessed relative to the lifted bass.
The most genuinely split axis, and it tracks the switches. In its default Harman setting the top end reads smooth, safe and inoffensive to most, with decent detail for the money — an easy recommendation for the treble-sensitive. Flip up the mid-treble switch (or play bright tracks) and a real camp hears it turn sharp, sibilant or harsh. ASR's measurement of the original backs the mechanism: all switches on overshoots the treble, and the dual-driver design shows a distortion rise in the lower treble that can read as edge.
Measured
The Audio Store's graph shows treble energy peaking near 2.5 kHz and again around 8–10 kHz. On the original Castor, ASR measured that turning all switches on 'caused a huge overshoot in treble' and flagged 'a pronounced rise in distortion in lower treble' from the dual-driver crossover — consistent with the reports that the top end can get hot or grainy.
⚠ vs. listeners — The graph and the 'smooth' impressions describe the default tuning; the 'sharp/harsh' camp is usually hearing the upper mid-treble switch engaged (or a distortion-tinged peak), not a different unit — so both camps can be right on the same earphone at different switch settings.
Where it splits
Smooth, safe and inoffensive by default — minimal sibilance, easy for long listening.47%
“Treble tuning is smooth with good energy”
The Audio Store
Sharp or harsh — instruments can turn piercing, especially with the upper switches on.53%
“The instruments get very sharp sometimes.”
kabir-doha, r/iemlndia
Soundstage
Moderate · 3 srcA recurring pleasant surprise for the price: reviewers hear a spacious, above-average-width stage with a more open feel than the original Castor, helped by the switch modes that trim the bass. It won't rival pricier sets for depth, but width and openness are consistently called out.
“Surprisingly spacious soundstage, with above-average width.”
The Audio Store
“Wide soundstage and excellent imaging for gaming”
RedditRecs (aggregated from 274 Reddit reviews)
“The pro's also offer a much more open and natural sound stage”
Solid_Steam, r/iems
Accurate and clear for a budget set, and a genuine draw for immersive, single-player gaming — the community aggregate even ranks it first for cinematic gaming. The caveat is competitive play: for pinpoint FPS cues the warm, bassy presentation is called subpar, so imaging reads good-for-the-money rather than surgical.
“Imaging is accurate and clear, especially in UUDD and UDDD modes.”
The Audio Store
“Subpar for competitive gaming”
RedditRecs (aggregated from 274 Reddit reviews)
Decent for ~$24 without being a strength. The default read is 'impressive for the price,' but the community aggregate lists a lack of clarity and detail among its cons, and at least one reviewer coming from the original Castor heard a drop in perceived quality. Call it adequate resolution, not a resolving set.
“It delivers impressive detail retrieval, especially for this price range.”
The Audio Store
“Lacks clarity and detail in mids/treble”
RedditRecs (aggregated from 274 Reddit reviews)
“Unfortunately, there is a noticable drop in my perceived sound quality.”
Solid_Steam, r/iems
The set's biggest fault line. The shell is large and the nozzle long and deep-seating — many find it bulky, heavy or fatiguing, it tops the community aggregate's cons, and a few units show driver flex. Others (and the retailer's own review) seat it securely and wear it for hours with good isolation. It leans clearly toward 'too big for smaller ears,' and a good eartip swap is near-mandatory for both fit and seal.
Where it splits
Too big — a chunky shell with a long, deep nozzle that many find bulky or fatiguing.70%
“it's considerably bigger than the already chunky Castors. It's longer, thicker, heavier, and the stem goes in deeper to boot.”
Solid_Steam, r/iems
Fine — a secure, stable, well-isolating fit for long sessions.30%
“No discomfort during extended use. These are comfort-focused IEMs with a stable fit.”
The Audio Store
A decent hybrid metal-and-plastic shell that feels better than its price — some even call it premium in hand — let down by KZ's generic braided stock cable. The recurring wrinkles are QC: the tuning switches can be fiddly and a few owners report driver flex, though most report the older QC issues have eased.
“It feels premium just like how the original castors did.”
Solid_Steam, r/iems
“The build quality is average, but it doesn't feel cheap.”
The Audio Store
“my unit had driver flex”
Altrebelle, r/iems (via RedditRecs)
The dual-driver setup — with a driver dedicated to the low end — gives it real punch and slam for the money in its bassier modes. Drivability is easy: at ~16–22 Ω the Improved-Bass version runs off a phone, though a small dongle or DAC tightens it up. (The original Castor was notably power-hungry; KZ says this version raises low-end sensitivity.)
“In UUDD and UUUU modes, the bass is powerful and punchy, with deep sub-bass rumble.”
The Audio Store
“This will drive your Castor Pro across any device”
AylaCODM, r/iems (via RedditRecs)
Measured
ASR measured the original Castor's sensitivity as the lowest of the Harman IEMs it had tested and advised 'a proper headphone amplifier'; KZ says the Improved-Bass Pro lifts low-frequency sensitivity, and owners run it happily off dongles — so it's easier to drive than its predecessor but still rewards a little power.
Isolation
Thin evidence · 1 srcNot much discussed, but the deep-seating nozzle and larger shell give it a good passive seal for everyday use — with the usual caveat that isolation, like the bass, depends on getting a proper tip fit.
“providing a snug fit with good passive isolation”
The Audio Store
Value
Strong consensus · 3 srcThe least contested point of all: at ~$24 it's near-universally called a budget over-achiever and an easy first 'real' IEM, with the tuning switches thrown in as a bonus few rivals offer at the price. The knock is only relative — set it up wrong (bad seal, wrong switches) and it can underwhelm — but the value verdict is emphatic.
“Exceptional value for money”
RedditRecs (aggregated from 274 Reddit reviews)
“Incredible value for money”
The Audio Store
“Go with kz castor pro improved bass edition. Bomb at this price.”
Acceptable_Hall684, r/iemlndia (via RedditRecs)
