Yeah, the fact that you are going out of your CD players analog output, and into your HK's analog inputs(rerouted from through your 2 channel preamp between them), which might be changing your analog signal back to digital, then back to analog(?...possibly), is surely not going to sound it's best anyway! Even if you're going straight out of your CD player's analog out's, dirrectly into your analog in's on the HK, It's likely still not that good!...as those analog inputs aren't as good as your preamps anway.
Regardless of the "feedback hum/buzz" issue, you should be forgetting about using your 2 channel preamp for what you are trying to do for surround, and just go digital out from your CD playr/DVD player or whatever, and go dirrectly into your Digital input on your HK!!!...just like you wold from you're DVD player! Then, you're still only selecting your "surround in" on your 2 channel preamp, just as before...understand?
I think the fact that you're trying to "feed" into two preamps, is already going to "crud up" the sound fairly bad anyway! However, if you insist doing it the way you are trying to do it(don't understand why, then you can play with "cheater plugs" w/no ground, and that might solve the buzz problem.
So, again, if you occasionally wan't to "process" your 2 channel CD sound into multi channel surround, just go digital out of your cd/DVD player into you're digital input on the HK, and flip the "processor" switch on your 2 ch preamp to let the pre/pro play through to your main speakers, as before. But, trying to go analog out of your CD player into your 2 channel preamp, then out to your pre/pro HK, looping back into your 2 channel preamps "processor loop" isn't going to work well!..infact It's the poorest choice you could do really.