This is a bit of a weird one since by altering the volume you are definitionally not doing digitally bit-perfect output; lowering the volume digitally (using the DAC) lowers the bit depth of the signal, so you theoretically lose some SNR. Generally it's easiest to think of it as 1bit = 6dB, so if you're at -6.0 dB on the preamp, you just went from 16bit to 15bit on your signal (if you started with 16 - it's a bit more complicated than all this but it's easy to visualize it this way). This is thankfully going to be inaudible, but technically you are altering the signal before it gets sent to an analog amplifier. We do this all the time with digital attenuation, for example if you lower the volume on your PC from 100%, you're also losing bit depth, however windows (in shared mode at least) resamples everything to 24/32bit anyway for the mixing and then samples it back down so that you don't get a bit-crushed effect, you just lose a bit of SNR which will be inaudible most of the time. In order to apply digital EQ for example, you always have to apply negative preamp so that your signal doesn't clip. Modern DACs will do upsampling of your signal so that you don't lose "information", only signal-to-noise ratio, so I wouldn't worry about it, just that I wouldn't call it bit-perfect when it's attenuated