But would you be degrading the signal if you were to do both at the same time? Since its not the same as lowering say your windows volume output which can impact the signal. I know its an extra step in the signal path, but passive pre-amps are very good at passing through the signal without altering it to any real degree.
My understanding is that power amp mode effectively hard-sets the burson volume to 99, and the signal skips the volume stage+opamp. And then you control the pre-amp input to adjust your listening volume. So from what i understand, theres two parts to how this changes the sound profile, #1 is you have removed the volume stage/opamp from the signal path, that is going to change the tonality of the sound. And the second change, is you are now providing more power to your headphones, because the amp is always running at full amplification capacity, you are just reducing the line level pre-amp volume that the amp is then amplifying to its fullest capacity.
I have read quite a few reports here that power-amp mode seems to be well received especially in power hungry planars, with people noticing increase impact, clarity, wider soundstage. But also noticing a more forward/sharp sound given the increase in power. Some people seem to find it a bit 'too much', and so im just checking if my understanding above is correct, because if it is, then i believe you could 'dial in' how much of the power-amp changes (that come from increased power, not op-amp being removed), by instead of setting power amp mode, instead just leave the volume on the burson on say 90 (as opposed to the 99 it would otherwise be hard set to in power-amp mode) and then continue adjusting the pre-amp level as you otherwise would in power-amp mode. Thus, you are dialing in the balance between pre-amp level and amp level to get a sound that may be more impactful, more clarity, then running in standard mode, but not quite to the point where it may be too forward or too impactful for your liking in straight power-amp mode.
I guess this is all just an exercise in trying to understand how power-amp mode works. I do know there are other amplifiers out there that allow you to switch the amp to act as a straight forward power amplifier. I believe its also common for people who use a pre-amp with an amplifier that has volume adjust to just set it to max and then control everything through their pre-amp.