According to a technical consultant at Emotiva, who I'd chatted with prior to my own purchase of the XDA-1, this unit was designed to run at its full 2v output (or "80" on the volume display) for absolute best sound quality. Apparently a setting down to "50" is acceptable, but lower than that and you start to lose detail and dynamics (again this is according to Emotiva).
I as well use the XDA-1 as a preamp in front of an Emerald Physics CS2 speaker setup, and what I've done is to set the amplifiers' input levels to where "80" on the XDA-1 is at a maximum volume I'd choose to listen to, and then use its volume control for attenuation. I typically listen at around the 70 or 75 (and as low as 65) settings, and have noticed zero degradation of sound quality (which I agree is quite good especially at its price point, and particularly since it comes with a very useful remote).
If you already use a separate preamp, then the above is moot - Just set the XDA-1's volume to 80 and you're covered.