I'm not familiar with your particular Denon, but most modern receivers offer 6ch (or more) pre-outs for precisely the purpose you mentioned. Use the Denon as your pre, add an amp, buy the 990 later. And yes -- if you choose to go the Outlaw route, those two components are all that's necessary.

M22ti mains, EP175 sub, VP150 center, QS4 surrounds