It would look cool.
I've been on the fence with the same decision but after reading a lot of info most folks, even Emotiva when pressed, say that an amp will not improve sound quality unless you're current set up is underpowered and clipping.
With the power of the 4520 and 2 powered subs providing power for the lf I doubt that an amp would sound any different other than letting you play a little bit louder.
I have an opinion!! Or as I like to say "Placebo man to the rescue"...An XPA-5 coupled to and powering the entire fronts of an 11.2 would be nice, After all, you can't go to 11.2 without another 2 at minimum. Or you could use the XPA-5 to power your FL, FR, Center, and Surrounds. Theory being those are the channels that would normally see the most wide spectrum activity and therefore need the added go juice of the XPA-5. I will also add that I owned 3 XPA-1s for my fronts and used two XPA-5s for the other channels; leaving me an extra 2 channels. I have recently gone on an amp weight loss diet and now have an XPA-100 for the center and the lowly 2 remaining XPA-5s. So, here comes the placebo. When watching incredibly dynamic and over the top engineered blow-em-up movies the current combination seems lacking as opposed to my previous 400 lbs of amplification. I have no scientific data to confirm anything but my system isn't quite as explosive as before. If I strip out and revert to my trusty 4311 (sorry no 4520 or 8801 yet), I drop another notch. This time noticeable in volume alone. I thoroughly am In the camp that old school class AB amps are better than the current AVR lot of internal amplification. Hugely better???? No, just marginally. But "placebo man" goes with HUGELY... It stirs more flaming and controversy.
So here are my pros to buy an XPA-5
1). You can't go to 11.2 without 2 more.
2). The XPA-5 is pretty cheap and you might as well have more channels than you need. Think buttkickers or whatever.
3). I perceive a more dynamic and powerful sound with my external amps.
4). Heck, the XPA-5 is pretty affordable and often on sale.
5). Ohhhhh more blue lights..... "Pretty"!!!
1). Seriously, for most room and most decently efficient speakers any old flea-bay stereo amp will get you what you need to go 11.2. As a case in point, Klipsch speakers are often very, very efficient requiring little real grunt to drive them to ear and nose bleed levels.
2). With great additional power comes great and ever more complex cabling issues. As the misses says regarding why she collects antique Federal style furniture as opposed to glass ware. Glass stuff "breaketh". Extra complexity is er........ complex.
3). Do you really need 11.1 or .2? OK, who am I kidding? Yes you want it. It's cool and I have enjoyed it for years now.
4). If married...... The need to have 11 speakers as opposed to suffering with only 9 can be a stretch. I say, buck it up though, pay your dues and enjoy for decades. My spouse is now numb to boxes, speakers, cables and assorted doohickeys. Frankly the rule we have is that "on average" everything can't be replaced for 3 years. Given my love of B&W Nautilus speakers, my oldest speakers serving Wide duty help me stay within the blissful marital guidelines.
5). As kind of hinted to before. If you have efficient speakers and a less than 15 by 15 by 9 foot listening area (just an example) and you do not listen much at reference. You probably don't need a big external amp. I happen to listen to very dynamic mutli channel movies at reference a lot and I like lots of volume and lots of reserve. Also, my fronts are not particularly efficient and my center is hard to get loud and hard to drive (IMHO).
Many people here will say the Denon internals are very good and all you need. IMHO, for most needs yes, but not for mine. I want ridiculous reserves and lots of power. Subjective, I am not!!!