iPhone 16 + Truthear SHIO + ATH-M50x: low gain is already enough
The SHIO's high gain gives 2 V and puts the M50x at 119.2 dB, 9.2 dB over target. Low gain gives 1.4 V and still reaches 116.1 dB, 6.1 dB over. The gain switch buys 3.1 dB of ceiling you do not need — and lifts the noise floor with it.
A $70 dongle and the most common studio headphone in the world: 116.1 dB on low gain, 119.2 dB on high, against a target of 110.
The M50x is 38 Ω and 99 dB/mW, which converts to 113.20 dB/V. Reaching a 110 dB peak takes 0.692 V, 18.20 mA, 12.59 mW. Truthear publishes two figures for the SHIO's 3.5 mm output: 1.4 Vrms on low gain and 2 Vrms on high. Both clear the requirement — low gain by 6.1 dB, high gain by 9.2 dB.
The pearl: the gain switch is not a volume control, and on this headphone the low position is the better one. Gain raises the ceiling and the noise floor together. The M50x turns 1 V into 113.20 dB of sound pressure, so it also turns the source's own hiss into something you can hear — this headphone is sensitive enough that output noise, not output power, is its usual complaint. With 6.1 dB of headroom already in hand on low gain, the 3.1 dB the high position adds is ceiling you will never reach, bought at the cost of a noise floor you might.
Power on the phone side is not the constraint. Apple's Accessory Design Guidelines put the iPhone 16's USB-C port at 4.5 W. Our record for the SHIO carries 2.5 W, and that number deserves a caveat: Truthear does not publish a consumption figure, so what is recorded is the USB 2.0 default bus allotment — 500 mA at 5 V — as a ceiling, not a measurement. The honest statement is that the dongle stays inside what the phone offers under any plausible reading, not that we know what it draws.
What we cannot check. Truthear does not publish an output impedance for the SHIO. The 1/8 rule wants below 4.75 Ω for a 38 Ω headphone, and there is nothing to compare against — so both levels above are computed with no divider loss, which is the optimistic end of the range. Practical note: the M50x arrives on a 3.5 mm plug with a screw-on 6.3 mm adapter, so it goes straight into the SHIO's single-ended jack; the 4.4 mm balanced port on the dongle would need a different cable, and Truthear publishes no voltage for it in what we cite, so we do not compute it.