I originally had a Denon 5803 for the downstairs using zone 2 for the outdoors, powering a pair of JBL Control 28's (8" woofer, 1" horn loaded tweeter) which are 92dB/w efficient and 8-ohms nominal. I had one per channel and used them for just some background music (like eating outside, not partying). If I'd leave them on, with the inside on plaything music, the Denon would go into protection if the levels were moderate. I checked cable gauge, finally ended up just getting a two channel Crown and using that instead, no issues since.
Oh, and my rack downstairs for the distributed audio has a dedicated AC duct, and a middle atlantic rack with a return air on top using the chimney effect. Lol, so, average temp inside the rack (I have the temp gauge unit from MA) is between 72 and 74 degrees even with the door shut.
It could be a poor design by Denon, or maybe I'm just expecting to much. I don't have another receiver to compare it to because my home theater uses a pre-processor and dedicated amplifiers for each channel.
I never tried running stereo on the inside, because main use of that downstairs receiver is 2 channel stereo across the left and right for nice even sound, and it does that well.
But, i'm with lsiberian. Get some pro sound amps and JTRs for parties. When I used to DJ I used my JTRs for both DJing and home theater lol. So, actually was a really good value. Just get the Pro ones (or at least the pro enclosure) so you have handles. Mine are T8s and are really easy to move. Actually my center channel is a stage monitor configuration.

Sounds the same, just easier to move (oh and you get nice Nuetrix speakon connectors which you'll end up wondering why you didn't always have those)