Is the PreSonus Studio 24c loud enough for the Koss GMR-540-A-ISO?
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.
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.