So I have a Denon 6700 and was considering a Bass X500 (or Monoprice 3) external amp to power the LCR channels. Am I hearing that I can use the LCR preouts from the 6700 into the Bass X 500 (or monoprice 3) to power my LCR channels and let the 6700 only power my surrounds, heights and rears. And that because I'm using the pre-outs on the 6700 for LCR channels, the LCR 6700 built in amps will not be running? ie: the 6700 will then run cooler and more efficiently? As a side note I was hoping to use the BassX500 to bi-amp the L&R channels and single the C channel. Am I out to lunch or will this work? Awesome discussions guys.
Yes you can use it with the A500 with the Denon's preout for the LCR. The internal amps will "not be running" in the sense that there will be no load connected to the speaker output binding posts for those channels, but they will still be connected to the pre outs so they will still be amplifying the signal voltage, just won't be outputting current, as again, there won't be speakers connected to those (LCR) internal amps. So yes, there will be less heat produced by those 3 amps, but the difference will be insignificant. You can reduce the heat if you then turn ECO on, but then it would limit the output to the other 8 internal amp channels, that may or may not matter depending on your conditions of use.
The 6700 has preamp mode, that would cut more heat, enough to be noticeable, but then all of the internal amps will be disconnected from the pre outs, and that means you have to use external amps for all channels.
I think the most economic way for you is to forget about saving heat, just spend $20 or less on an external fan and problem solved.
Regarding passive bi-amp schemes, there could be audible benefits under certain conditions but in general and in most cases (including yours), it won't make any difference. You would be better off going with the A300, that at lease offer a little more output than the Denon. The A500 actually offers less output than the Denon, and higher distortions/noise.
That amp is relatively cheap so good value but on performance basis, don't expect it to "sound better" than the 6700's own amp, probably worse if you have excellent hearing ability to discern minor/subtle difference due to distortions and noise.
There are bench test results (just in case you believe in specs and measurements) for some Emo amps, including the BassX A500:
Emotiva BasX A-500 5-ch Amplifier Review | Audio Science Review (ASR) Forum