I have tinnitus and I have to listen at low volume like 50dbA. In this case would a tube headphone amplifier still offer the tubey/blissful sound that it is known for ?
Or would it not shine unless the volume is higher ?
Louder being better is due to a number of things:
1.
Psychology, ie, humans very generally tend to perceive louder being clearer and better even with measurable distortion so long as it is not outright bad distortion or very audible noise. In short...if your brain is still working off this principle, then no amp nor any other hardware will help you. Maybe software can, like Night Mode on a home theater receiver. Or EQ...just boost both ends, because human hearing has a bias for the midrange freaquencies, biologically (ie why we measure only 20hz to 20,000hz) and psychologically (ie why there's a Harman Curve target which is an attempt to compensate for this, among other things, like ambient noise).
2. Possibly related to the above..
.playing at a louder volume covers up ambient noise. The louder the material you are listening to is, the less ambient noise, the better the sound to the listener, or the louder the low end to get past the ambient noise...until you hit really bad noise and distortion, or outright clip the signal. If your room hasnoise generating components in there, like a non-split type A/C or a fan or you crank up the A/C's fan, or heck, a computer with fans, good luck running music with practically no isolation if the computer's already running at, say, 40dB therefore making the room ambient roughly around 40dBA, minimum. And even without such equipment a regular room that isn't out in the boondocks could have as high as 40dBA already (and the only real reason some people can still hear six BeQuiet or four Noctua fans is because what a mic measures as the loudest with these not running and these running might be at different freqs, so once these do run, you hear them). If it's a gaming rig without a quiet loop, good luck with 80mm or 92mm fans on older graphics cards; if you got a new graphics card with the bigger coolers, let's hope it's not Gigabyte with the elevated TDP and voltage (then again you can crank this down in software) or the last gen Asus AMD card that has the wrong clamping pressure on the cooler. Or maybe you have a quiet loop but the pump is audible. In short...this can be very, very, very difficult to deal with to get to the point that would be helpful for your purposes outside of 1) building a dedicated listening room that is acoustically isolated (audio engineering lab or serial killer torture basement level acoustic isolation, ie, either the neighbours can't hear the girls screaming or so quiet you can hear your own heartbeat) that only has your audio gear in it (ie so you can get close to hearing nothing but your own heartbeat with the headphones off).
3. Cranking it up makes the amp's distortion more audible, but
in some cases the amp's distortion might be euphonic, ie, instead of getting a very clean 0.0001% THD+N and 500mW per channel, you could be getting 0.5% THD+N out of only 200mW but you're introducing an EQ effect to the sound and that will be more apparent. Your problem here is that it will also increase the noise, so you need very quiet tubes, and on the rectifiers, tend to also have lower gain.
4.
Or the amp simply has a lot of euphonic distortion, so pick an amp that will go Harman Curve with the load of your headphones. In short...use a low impedance headphone and a specific OTL tube amp that will react that way to that headphone's impedance (note: not all OTL amps will be similar, much less the same, in that regard, because the effects of low damping factor can vary).
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Another option: just skip the tube amp and get an in-ear monitor, wear ear protection over it to increase the isolation layers, and then power it with a very quiet DAP with a lot of attenuation so you can fine tune the loudness without jumping from "almost there" to "STOP STOP STOP TURN IT DOWN MAKE IT STOP I CAN'T TAKE IT I CAN'T TAKE IT I CAN'T TAKE IT OH OH OH THE BLEEDING ME I AM THE BEAST THAT FEEDS THE BEAST I AM THE BLOOD I AM RELEASED COME FEED ME A CURE GIVE ME A CURE I'M CAUGHT I'M CAUGHT I'M CAUGHT UNDER CAUGHT UNDER WHEELS ROLL OH BLEEDING EEEEEEEEEEEEEEEEEEEEEEEEEEEEEARS" with just one press of a button (like my old SGS3 when driving the 120dB/1mW Aurisonics earphones). And yes not having a Death Magnetic loudness war problem will help so you can keep the output low enough without getting channel imbalance, something that is easier to avoid on a DAPw with digital volume control than on a tube amp with an analogue potentiometer.
