So to be clear, using the Disabled Volume setting has no effect on internal processing whatsoever (e.g. using ReplayGain or downmixing surround) it's just that it is preferable to use the internal volume to another control somewhere else on the PC like the system mixer.
I mean, that's what I expected to be the case, when DSP Studio reports the internal precision as 64-bits regardless. The way things were worded on the Wiki is somewhat confusing and seems to imply that using Internal Volume is somehow better than Disabled Volume, even if I were to control volume externally. (I adjust it in the amplifier)
It's too bad that you can't have the PC control the amplifier volume level for things like ReplayGain.
Internal volume is superior to disabled volume (and affects DSP) to the extent that you use any DSP that boosts gain in any way (EQ boosts, linkwitz transforms, convolution, etc.). If you have volume disabled (and maxed), or have internal volume set to 100% and any DSP effects apply gain, peaks will engage clip protection, which is undesirable (lowers volume during program material, affects dynamics, etc.).
If you use internal volume as your main volume control, you'll rarely have it set to 100%, so will almost always have headroom to apply those DSP effects (and you can set your internal maximum volume if you want to make sure that you never risk engaging clip protection). That's why the wiki suggests internal volume is preferable. For example, I have my amps opened up and use internal volume as my primary system volume control. This allows me to use a linkwitz transform without fear of clipping.
If you always leave internal volume at 100%, there's no difference between the two, but if you use any gain boosting EQ, internal volume set appropriately below 100% would be a better idea.