Yesterday I did a subwoofer torture test by playing "Bass I Love You" at high volume, and this track had no problem sending the Crown amp into clipping mode. That's the only track I've found so far that makes it clip at high volume, but I also haven't tested that much either. The Dayton subs are rated for 300W RMS and 600W MAX each, and the Crown should deliver a max of 750W to each sub pair into 2 ohms, so about 375W each. As such, I'm not terribly surprised that it clips under these circumstances, especially considering the 2 ohm load.
Also, under more focused listening, and moving back and forth, I can tell the new rear subs are ever so slightly behind the front subs in time, and a little bit of delay between the front and rear would be beneficial. It's fairly subtle, and almost sounds more "spacious" but if I roll my chair back to fall perfectly between the front and rear subs, it noticably tightens up. The rear subs are 3 feet further from my ears, so I need to add 3ms of delay to the front mains and front subs to let the rear subs catch up.
I could ignore the clipping problem since it seems to be isolated to fairly extreme examples, but now that I've keyed on the delay issue, it's annoying me. The fix requires buying another miniDSP to add a little delay, but as a bonus this opens up the ability to DSP each sub individually. This also requires 2 more amp channels, which will take care of the clipping issue as well. So for my next move, I just picked up another Crown XLS 1500 off ebay, and I also ordered a miniDSP 2x4 HD.