preamp-poweramp hiss (loud)

I have a PAT-4 STEREO-150 Dynaco setup. When units are hooked up together, and balance control in center position, hiss is very loud. With balance far right or left, no hiss. I hooked power amp up to another preamp, no hiss. Both units have 2-prong plugs. What am I doing wrong?
I used to have a PAT4 & it was rather noisy. Can't explain what's happening re: your balance control affecting that though? I did replace all the stock resistors with low noise metal films of identical values, which helped alot.
Try hooking units to different circuit (from different breaker) in house. A monster 3500 helped when I had similar problems w/ amp. Get the balance control checked. Only thing if it is not the balance, you have left and right out of phase with each other. Good luck. I really would go to good dealer/shop with this ??