'As discussed, lowering the volume in software ... is accomplished by reducing bit depth.'
No, reducing the amplitude of a digital signal does not change its bit depth. If the amplitude of the digital signal is reduced, then a couple of bits on the MSB side will be constant. However, reducing the bit depth would make bits on the LSB side constant.
To state it differently, a lower bit depth increases the quantization levels and results in lower quality audio. Reducing the amplitude retains the same quantization level, and therefore has no reduction in audio quality (At least, not due to bit depth, it does change the SNR as you suggested)
You're correct that lowering amplitude ~= reducing bit depth. I'm not sure that lowering amplitude digitally keeps S/N constant but I'm a bit out of my depth (excuse the pun) at this point.
To further muddy the waters, in most cases using software volume control won't noticeably reduce audio quality. The main point I was trying to make is that on most computer setups 100% volume should be the least noisy, most accurate signal that it's capable of.
No, reducing the amplitude of a digital signal does not change its bit depth. If the amplitude of the digital signal is reduced, then a couple of bits on the MSB side will be constant. However, reducing the bit depth would make bits on the LSB side constant.
To state it differently, a lower bit depth increases the quantization levels and results in lower quality audio. Reducing the amplitude retains the same quantization level, and therefore has no reduction in audio quality (At least, not due to bit depth, it does change the SNR as you suggested)