I registered just to add my voice to the chorus of "MiniDSP 2x4HD sounds noticeably bad". I can't believe how many people are trying to make it sound like you're crazy for hearing it. I've tested with several speaker/amp/dac combos, inserting it - with no processing engaged - in between the DAC's analog output (volume controlled) and the amps (or active speakers).
Issues:
1. Distortion from bass filters. There are many ways to set the bass filters such that they produce measurable and audible distortion. All of these are perfectly legal from using the MiniDSP GUI - I'm not even talking biquads imported from REW (which distort even worse). Yes, I made sure there's no clipping anywhere, and this behaviour is exhibited even with only cut filters.
2. Hiss. Very audible in this scenario (since miniDSP output is being amplified at full gain).
3. Loss of clarity / punch. Yes, this is real! It doesn't sound outright "bad" - just "worse than without it". To me, that makes it unusable. This is less of an issue when you use the MiniDSP digital inputs, but its still bad.
It's a hard scenario to get right because MiniDSP converters are dealing with really low input and output signals, and full-scale amplification of the unit's hiss. But this is the exact scenario that the MiniDSP is designed for! We aren't going to use a 4-channel analog volume control after the MiniDSP to trim down the hiss level and allow higher input and output...
So, although it offers fantastic flexibility for integrating subs, or a 2 way crossover with correction, the degredation in quality eventually gets to me and the MiniDSP returns to the drawer, to be replaced by an analog crossover (plus software room correction).