My advice is to skip the Denon and get something that has Dirac Bass Control. That cab independently equalize four subs. That will make a much bigger difference than the brand you choose. In fact, stop looking to spend so much on subs but rather look for something that has Dirac Bass Control. The Monolith HTP-1 has Dirac Bass Control with enough outputs to control up to five subs. Too bad it's not going to be in stock for a while. Arcam's AVR series has Dirac Bass Control.
Here is something that should work great for that.
If your friend cares about sound quality, stay away from Audyssey and go for Dirac Live with Bass Control. The subwoofers won't matter as much, in fact, go for a cheaper sub to get a more expensive AVR. You might just look at getting four Monolith 15" THX Ultras. That will give you all the output you will ever want and terrific sound quality, and four of them with Dirac's Bass Control should be able to net you pretty much a perfect sound so long as you use some good sense in placement.