parallel receivers

The problem that you are experiencing is due to the fact that the way you are wiring them up, the impedence the amp is "seeing" is effectively cut in half and the most receiver amps are not up to providing adequate power into a low impedence load. The issue here is how the speakers are hooked up, not the amp, per se. Others will give you a more proper technical answer, I'm sure.

When you talk about wiring do you mean the way the speakers are connected to the receiver? Right now they have the single prong banana plugs, but I could concievably do some sort of bi-wiring. Would this help?

Thanks

No, its whether the speaker outputs are wired in series or parallel. FOr most receivers with two sets of speaker outputs, they are wired in such a way (can't remember if its serial or parallel) that if you switch them both on at once, the effective impedence that the amp sees is cut in half, and if the speakers originally are 4 ohm and dip lower, you will have a receiver with a limited power supply trying to drive what is effectively a 2 ohm or lower load. Most receivers and many power amps cant to it. Biwiring has nothing to do with it.

My experience ( with several units from the 70s and 80s--Yamaha, Marantz, HK, Sherwood, Akai ) is that very few receivers have the kind of dual outputs you are looking for... I haven't seen even one. Someone else here with more experience may be able to give you a name, though.

One alternative solution which is sometimes proposed is to buy a second amp ( or a second receiver ) for the remote speaker pair, and run an interconnect from the tape outs of one to the tape ( or aux ) ins of the other. There is a risk of hum with very long wiring runs, so well-shielded wire is a useful precaution, but a few setups I know of have worked this way.

How about picking up a impedence matching speaker selector box. Niles audio sells one

http://www.nilesaudio.com/products/aip_speaker_selection.html

This will allow you to hook up multiply pairs of speakers to a receiver, or amp, and the receiver will see a constant ohm load. I have a friend that has 8 pairs of speakers hooked a Denon receiver for his whole house audio system. I've been over his house for a few parties, and it has plenty enough volume.

Good luck,