need help with tweeter hiss

I just got some new amps to try out in my home theater. They are (5) Gamut monoblock m250's. Before that. I was running a legacy monobloc 5 (a five channel amp made by coda). I never had a tweeter "hiss". Now I am hearing a "hiss" from my Tweeters with the Gamut's hooked up. Specifically, when I turn on the amps, I hear a slight "hiss" (I believe that to be normal) then I turn on my pre-pro the "hiss" is louder(even audible from listening area). The "hiss" does not increase with volume. It is coming out of ALL five speakers. All the interconnects are balanced from amps to pre-pro. ANY suggestions? Some people are telling me that it could be the gain (amps or pre-pro) or the interconnects( i have tried switching them around). Or the amps matching up to the speakers. The rest of the system is: Lexicon mc12b, b&w 802 nautilus, 805's, & htm1. I did try searching the archives but nothing there. Thanks for the help.
If your new amps are much higher powered than your old ones and do not have a very low internal noise level (signal-to-noise level), you may be hearing the residual noise of the new amplifiers. But that would be very rare in any of today's amplifiers that boast s/n ratios of about 100 dB. I guess I wiuld check for clean connections next.
It's the amp causing the noise. Although it's not really desireable, it's not really abnormal either. If it's really low level and doesn't increase with volume, I wouldn't worry about it.