I've always wondered this though, I have an Excelon too, in the menu you can turn off the built in amp, I did this and my bass played but the door speakers didn't work so I figure you turn this off when your speakers are being powered by a seperate amp and maybe that internal amp causes the early clipping?
I read a while ago that the sub level needs to be at +15 so I did this and it made my subs come to life, no issues at all with my subs and its been about 2 months now. I'll have to maybe talk to Treydog on here since he lives near me and has a DD-1 so I can get mine tuned and see what happens since my x592 is an older model.
I usually have my volume at 28-30 anyways