Amplifiers do not mind having different loads on some channels. It’s quite like lamp circuits. You can keep adding lamps until the circuit limit is reached and the lamps do not interact with each other. There is no need to have exactly the same number of lamps on each circuit in the home.

Two pairs of 8-Ohm speakers, wired in parallel, present a 4-Ohm load and AMP is rated for 4-Ohms. There is, however, one slight nag with connecting two NHT O2-ARC speakers to a channel. This speaker specifies a dip to 3.5-Ohms at an unspecified frequency. AMP is tolerant of impedance dips below 4-Ohms, but running two of these speakers on a channel might be pushing things a bit. Probably you’ll be OK because the speaker wire will add some protective resistance.

Run a test by connecting both of your speakers to one AMP channel and play some music at your loudest listening level.  AMP will protect itself and let you know if this is not acceptable.

Hi buzz,

Thanks very much for the response. We have neighbors pretty close so the volume never goes about ~40-50. I will test it out before buying and wiring a third speaker though. I'm assuming this test would give me good information about using either 3 or 4 if these particular speakers with the AMP. How long should I consider an adequate test time?

For the protection circuitry probably 10-15 minutes should be adequate.

If you are checking for heat issues I’d just monitor the Amp’s temp as you run the test. If it stops rising without shutting down or reducing power you are likely good. If it is still warming up run the test until it stabilizes.

I’d also try a variety of music styles. You’re trying to exercise the widest swath of extremes for the Amp, to see how it handles things. Don’t stick with just one thing. 

As suggested, play a variety of music. It would be a good idea to include some thumpy bass.

Thanks everyone. I hooked both speakers up to one channel and ran it at a level that made me feel a little bad for the neighbors for 30 minuets playing a variety of music. The AMP itself felt just a little warm to the touch and an IR thermometer showed mostly mid 90s but 102 F at the hottest point. Its mounted vertically on a wall in the unfinished portion of our basement with nothing else around it. Ambient is usually no hotter than high 60s. Not sure, but that seems to be within an acceptable range to me. Please let me know if it’s not.

The volume with two speakers (I was played mid 60s to mid 70s on the app) was plenty loud for my needs. I think I’m going to stat by moving the existing two speakers to a more favorable location but leave a spot where a third could easily be added and run wire for that one at the same time. That way adding a third if needed will be a simple project. Two might just be enough. However, I now know that I CAN add a third so thanks very much to the group for your help.

Cheers!!