Koss GMR-540-A-ISO and GMR-545-A-AIR publish the same 15–22,000 Hz — closed and open both
Every figure on the closed ISO page matches the open AIR page: 35 Ω, «Sensitivity: 103 dB SPL» with no unit, 15–22,000 Hz. On a PreSonus Studio 24c the milliwatt reading gives +11.7 dB and the volt reading −2.8. Koss writes «/1mW» on its own Porta Pro page.
Signal Chain Compatibility Matrix
Select gear and parameters for live gain, impedance and SPL calculations.
Input Equipment Setup
PASS: Optimal Signal Chain
Requires minimum +60dB of clean pre-amp gain. Most budget entry interfaces max out at +50dB to +56dB and introduce hiss at max pot. An inline preamp booster (Cloudlifter CL-1 / FetHead) is strongly recommended.
The interface preamp provides sufficient headroom.
$538
$405
-25%Tonal & acoustic characteristics: Flat, wide-range frequency response for clean music and speech. Smooth, warm acoustic character with air-suspension shock isolation.
Technical advisory: With +69dB max gain, the Gen 4 Solo can drive demanding dynamics like the Shure SM7B natively without requiring an external inline booster.
Physical Interconnect Topology
Shure SM7B
Direct Cable Connection (No Intermediate Box)
1x Shielded Balanced XLR Cable (3-Pin XLR Male to XLR Female)
Focusrite Scarlett Solo (Gen 4)
Host: Apple MacBook Pro (M1/M2/M3/M4)
An open and a closed enclosure cannot measure the same frequency response at the bottom of the range. Koss publishes 15–22,000 Hz for both, which means at least one of those sheets describes the driver rather than the headphone you buy.
The GMR-540-A-ISO page prints «Impedance: 35 Ohms», «Sensitivity: 103 dB SPL» and «Frequency Response: 15-22,000 Hz». The open GMR-545-A-AIR page prints the identical three. PreSonus publishes for the Studio 24c «47 mW/Channel @56 Ohm», which is 1.62 V, and no output impedance at all.
Read per milliwatt, which we store. S_V = 103 + 10·lg(1000/35) = 117.56 dB/V. A 110 dB peak needs 0.419 V, 11.97 mA and 5.01 mW. The Studio 24c reaches 121.7 dB SPL, +11.7 dB, verdict comfortable — with the caveat that PreSonus names no output impedance, so the damping ratio is unknown and this level is an upper bound.
Read per volt. The peak would need 2.239 V, 63.96 mA and 143.2 mW, and the same interface would give 107.2 dB SPL, −2.8 dB, too quiet. The missing «/1mW» is worth 14.6 dB here, and on this particular source it is worth the verdict as well. Put the same headset on a FiiO K11 (4.24 V) and both readings pass — +19.8 dB and +5.3 dB — so whether the ambiguity matters at all is decided by the source, not by the headphone.
How to judge. Koss writes «101 dB SPL/1mW» on its own Porta Pro and KSC75 pages and drops the «/1mW» here and on the KPH30i. The house convention is explicit elsewhere, so we read per milliwatt. The alternative would make this a 88.4 dB/mW driver — implausibly deaf for a 35 Ω gaming headset, and out of line with everything else Koss publishes. As a sanity check, the KPH30i (60 Ω, 101 dB/mW) computes to +7.4 dB on the same interface, which is where a 60 Ω on-ear should sit.
The pearl: the frequency response is the giveaway, not the sensitivity. Sensitivity and impedance can legitimately be shared by two products built on one transducer. A frequency response cannot: an open back vents the rear of the diaphragm and rolls off the low end, a sealed cup loads it and extends it. Identical 15–22,000 Hz on both is therefore a driver datum copied onto two product pages, and the honest use of the whole table follows from that — good enough to size a source, useless for choosing between the two models. On this interface both answer +11.7 dB, and they must.