BTW, is there any way to bargain on that 30db noise floor? Can we find out the max recommended level for a recording studio and go with that number? Granted nobody lives in an anechoic chamber, but 25db seems easily attainable, and 30db is more than 2x as loud as that.
Studios are built to meet NC curves (Noise Criteria) which are based on noise audibility vs frequency. When octave band noise is measured the SPL reading is plotted. The NC specification is the curve below which all octave band readings fit. One high band can bump the entire reading up.
NC-20 is good, there are many studios at NC-15, and a few at NC-10. The NC figure is almost always impacted the hardest by air handling, but outside impinging noise can also be a factor, such as trying to build a studio next to a 4-lane highway.
Home living rooms easily do NC-30, NC-25 isn't all that unusual, and there are quite a few at NC-20 between air-handler cycles.
To make this meaningful in a headphone system the noise presented at the ear would be what's important, and would be calculated as the result of system noise, total system gain and headphone/IEM sensitivity. Plotting this against NC-type curves might seem to be relevant, but in reality electronic systems have greater noise at higher frequencies, so higher tolerance of LF noise would be unnecessary. Some sort of noise weighting would make sense though. A weighting might be reasonable since it seems to persist, but wouldn't it be cool to use ITU-R 468? So, scale the noise spec to SPL with ITU-R 468 weighting. Could spec "ears" with minimum noise at 0dB SPL with the same weighting, which is technically wrong, but easy to do and frankly close enough.
Footnote: It's a small point, but a 10dB change is perceived as either doubling or halving the volume even though a little more than 6dB is a double/halving of pressure/voltage and 3dB doubling/halving in power.