Well, several variables changed so hard to tell which made the difference. You could put it back in the rack and try playing the stereo system at the same volume again. If it shuts down then I guess you know the answer, heat/air flow is the culprit. If OK, then ad back the other speakers and try the lower volume experiment. We don't have any experience with the Denon receivers so we can't be much help here, perhaps other forum members have some experiences that can shed light on the problem. (I can tell you we haven't really heard of ANY receivers having troubles like this driving TritonCinema Two systems.)

Make sure the speaker wires didn't touch the receiver body cause it will shut down

I measured the resistance across the SuperSat 3 and noticed it read 3.9 Ohm. The Triton 2 seemed to be 8-9 Ohm. Are these the expected resistance measurements? Is it possible that I am overdriving the Denon after a long session at -6 dB reference? After several hours of higher volume listening I think it just can't keep up anymore.

You've measured the DC Impedance of the speaker, not a relevant measurement.

If the Denon can't drive these speakers then there's something wrong with it, really. Or else it is just not a good piece of equipment (sorry, not many Denon users on this forum so we don't have much experience to relate - that alone probably speaks to whether it is the best choice for your Triton-based home theater).

Any recommendations on what to buy that is better? I am open to different options! I'm sick of Denon at this point. What do most people use to drive a system like this for home theater use mainly?

I'm sure you'll get plenty of feedback to that question from the gang here.

As far as receivers go, we have used and recommend models from Anthem, Arcam, Marantz, NAD, Integra and Pioneer Elite. I'd suggest you consider models in the $1K-$2K price range.