Help me out... can't figure this out

Hey guys,

I have an old system of Rotel sound processor (rsp-985) and 5-channel amp (rb-985), which functions fine together. Recently, I tried to add a Rotel rb-990 (2-channel amp) to the above HT-system and eventually do a 7.1 when I upgrade my sound processor.

My initially intentions were to use the RB-990 to power the front speakers and 3 of the 5 channels from the RB-985 to power the center and back two speakers and leave the remaining two channels for a future 7.1 system.

What happens is that when I connect the RB-990 to the "front L/R" preouts of the sound processor and turn on the amp, there is a horrible constant loud noise from the engaged front speakers. It was so loud, I thought I blew my speakers out, fortunately they were fine.

When I connected the 2-channel amp to the "surround L/R" preouts from the processor I didn't get this problem.
Also, I switched out the interconnects and still, same problem.

When I ditched the 2-channel amp and connected the processor to the 5-channel amp (the original configuration), it worked fine.

I don't know why I am having this problem, especially since I've connected the 2-channel amp to the existing system in the past with no problems.

I've narrowed the problem to the "front L/R" preouts of the processor and it's compatibility with the 2-channel amp, but independently neither seems to have a problem when I switched the other out.

Any advice would be greatly appreciated.

Thanks.