I think the only difference is the amps. They all have the same chips\Dac. The 1120 has Toroidal Power, while the 720 doesn't. All 3 from 520 to 1120 are rated 140 Watts for fronts 60 for surrounds.
I use a ADA 1250 3 channel for my front stage. Limiting myself too 5.2.2 at the moment with my MRX720.
Out of the box the 720 is 7.2 or 5.2.2, The 1120 has 7.2.4 and the AVM 60 has true balanced XLR output. To which is the reason for the AVM 60s higher price, it doesn't have the amp's though it does have more circuitry inside.
On that note if you can all of us Axiom\Anthem heads highly recommend. The ADA's and all speakers play really nicely together.
I intend to stick with my 9 channels, and possibly play around with adding two more, so I think I should stick with the AVM60, move to the 1120, or get one of the Denons......
My room is just too small to add side surrounds at the ear level. No way to walk around them...... That's why I have QS8's mounted close to the ceiling (using Axiom ceiling mounts). My surround backs are Q's mounted close to the ceiling as well, but I do have room to use a set of M22's I have directly behind the seating - about 3' behind anyway.