I forgot to mention that power amps are needed coz though the X3700H is capable of processing 11 ch it's only a 9.2ch 8K AV Receiver meaning one needs to externally power 2 channels to do a 7.2.4 setup. This is the primary reason why an external amp is required/needed.
In your other thread you are asking about the Rotel RMB 1585. That amp looks beautiful, but for a lot less money you may want to take a look of the Hypex based amps by Buckeye.
Buckeye Hypex based power amps
The NC502MP based one is much more powerful than the RM 1585.
4 channel NC502MP, XLR inputs (Neutrik), Mogami wiring, 12v trigger, Power LED (Blue) =
$1049 shipped
You should be able to get him build you a 3 channel for under $1,000.
The AVR will most likely be able to power your other 8 speakers with ease. Have you figure out your predicted output power need yet?
The RM1585 is probably not a good match to the Denon, not because of its low gain, but its low input impedance, unless you use all external amps.