hissing sound from krell showcase

I have a krell shocase with a Mark levinson amp, I get perfect sound from my cd in preamp mode but get a hissing sound when I use front plus sub mode. I used 2 different subs with the same result any suggestions would be very helpful. I use XLR cables to both the amp and the sub
Do you have your Krell hooked up to your tv and/or cable box?
the hissing comes thru the main speakers and the sub? both main speakers? disconnect sub and turn on front plus sub mode anyway...still hissing? just curious if you can figure out if it is some kind of circuit problem in your electronics...or a connection problem.
Is your Krell hooked up to your tv/cable?