Separate Amp for AV receiver

What is the amp?  I mean if it is an old Krell or Bryston that the OP is calling cheap, it might make a difference.  The ultimate question here is the separation of pre amp and power amp.  For the sake of argument let’s say we are in the 2 channel world.  Someone has a cheap pre and a super duper power power amp.  Now in theory all that the pre is supposed to do is pass the signal unchanged from the source to the power amp, and the character of the latter will determine the sound.  In practice, we know that the pre matters very much, and most of us budget roughly equivalent for pre and pro.
  I would argue that the pre is even more important in multichannel, because the challenge of reproducing a realistic—as opposed to gimmicky—soundstage in multichannel.  So to cut to the OP specific question, and having actually owned that Denon for a brief time, I wouldn’t bother.  It will be the lipstick on the pig phenomenon.  The Denon is dry, desiccated, and it could be exhibit A in support of Millercarbon’s post.  No power amp will fix what is wrong with it.
  I disagree with MC in that while separates are to be preferred, it is possible to get pretty good sound from a receiver.  Good room correction helps, and again I think RC is more important in multichannel because some of those surround speakers can blow up a soundstage unless they are properly controlled 

I like AVRs with room correction.  I would check out Anthem or NAD, which uses Dirac.  RC can really make a difference with mid priced gear.
  I didn’t pay anywhere near $3K for that Denon—more like a Grand, and that was new

OP, you have a nice 2 channel system, but you can’t compare your mid Fi HT components to premium priced 2 channel equipment and make a valid comparison.  It’s like expecting the same performance from a Hyundai Sonata and an upscale Mercedes Benz.  Since you like McIntosh why not get a receiver or Pre/Pro from that company and then make a comparison?