Onkyo SR876/NHT 2.5i front speakers??

I would first try "bi-wiring" the 2.5i's. This will greatly improve the overall performance of your mains, while affecting them as more efficient, as you'll get allot more current to them. NHT recommends bi-wiring / not bi-amping. That's the main reason they provide the double connects. Unless you run "sub out" low frequencies to a seperate power amp for the two woofs (stereo) that could also provide much better bass definition, and you would not run into the timing issue mentioned above, or volumn control issues. Experiment and have fun.

RE Audyssey
It should not matter, from what I understand.

RE biamping
selecting biamping defeats two of the surround channels, you do know that right? Aux zones too will become affected as the result.

Your path to do it trys to keep the 7.1 audio in tact, yet I'm thinking No. Not that way. But don't let that stop you from trying.

I would think for biamping the fronts while keeping the audio in 7.1 config, I'd use a Y pigtail and run two sets of cables off it to two amps directly., or to one four ch amp and then to the speakers.

aNOTHER PROBLEM YOU MIGHT RUN INTO IS THE TIMING OF THE SIGNAL GOING YOUR WAY, USING TAPE INS AND SUCH.

Feasible? yep. Worthwhile? Questionable.

IMO? Get a 7, 5, or even a 3ch amp instead and use it/them. They'll surely do better than the onboard amps.... and just how hungry are those NHT units ANYWAYS?

Even a nice two ch amp for driving the fronts will be more beneficial... all around. The rec will work easaier, and the mains will have more juice. And it's keeping thinngs simpler.

Good luck.