I finally got a chance to listen to the XD 400/4 side by side with a Kenwood XR4S, Zapco DC 360.4, and JL Slash 300/4. I can say that you could tell the difference between all of them. The XR4S was on the bottom not so much because of the sound of the amp but because its fan is damn loud.The was a slight edge to the sound which is typical of a class D amp. The XD was next. The edge was less noticeable on this amp but it was still there. Unfortunately, there is no magic that will get a class D amp to sound like a class AB amp so there was a noticeable difference between the XD and the DC 360.4 and 300/4 which are both class AB. The difference between DC 360.4 and 300/4 was much harder to distinguish. Based on just features alone, I would opt towards the Zapco because the DSP would make a huge difference when doing the final tuning if the system didn't already have a DSP.
09 E90 M3 - Interlagos Blue / Black Novillo / Sycamore Anthracite / Eibach / HRE