I have been giving this setup serious consideration also. I have read (mostly in the Audyssey thread) that in many cases the XPA-3 makes only a subtle difference, often dependent on which speakers are being used. In my case, I am filling a large HT room (20' wide by 30' deep) with large DefTech towers across the front (BP7002's and a CLR 2500), plus Heights, Wides, Surrounds and Rears. Currently, I am using a left over Pioneer 110w AVR as the amp for the Heights. It seems to work fine, but I keep thinking I might be better served by an XPA-3 taking over the 3 large fronts and letting the Denon 4520 breathe even easier on the remaining 8 speakers. While it's easy to suggest previewing, in reality this is hard to do. I will have to check Emotiva's return policy on that possibility.
As for the Dialogue Enhancer - I have the same experience describes by AJ. My center channel has always sounded good and the Enhancer in my system simply seems to make the dialogue louder. Like LFC, I can see how it would benefit those whose speakers are struggling or less clear, but in my system both features are unnecessary. This doesn't diminish their value overall, just relative to my needs. LFC seems like a very decent solution, but for a problem I don't have. Same for Dialogue Enhance.