Hissing sound coming from tweeters

No, the hiss won't cause any damage.

The MC252 will reduce the 32db gain of your present amp to roughly around 23db, according to my calculations. Not sure that will be enough of a reduction to be satisfactory. Also, using a 250 watt amp with 101db speakers most likely means that you will be paying for a lot of watts that you'll never use, unless you are envisioning going to less efficient speakers in the future.

Regards,
-- Al

Id rather have too much power than not enough. The specs for my speakers indicate that the power handling is 250W RMS. I don't want my equipment to have to work too hard to get some serious sound out of them. Al, how do you calculate the reduction in gain, and how much reduction do you think is needed to obtain satisfactory levels?

I can hear the hiss 7 ft away from the speakers :( Anyone want to buy my amps?

Al, how do you calculate the reduction in gain?

As a rough approximation, I neglected dynamic headroom and calculated the gain for the MC252 based on the continuous maximum power rating of 250 watts. Based on P = (Esquared)/R, 250 watts into 8 ohms corresponds to 44.7 volts. The specified sensitivity for the balanced inputs is 3.2 volts. Therefore the voltage gain, neglecting dynamic headroom, is 44.7/3.2 = 13.97. Based on 20log(Vout/Vin), that corresponds to 22.9db.

I now see in one of the spec sheets that dynamic headroom is specified as 1.8db. So depending on what power level the sensitivity spec is defined with respect to, the gain may be as much as 22.9 + 1.8 = 24.7db, which is only 7.3db less than the gain spec of your present amplifier.

how much reduction do you think is needed to obtain satisfactory levels?

A commonly cited rule of thumb is that a 10db reduction in volume is subjectively perceived as being about "half as loud." Based on that, and on your initial description of the present hiss level as "very annoying," my rough guess is that you would want to reduce the amplifier gain by significantly more than 10db with respect to what it is now, perhaps reducing the present 32db gain to the area of 16 to 20db or thereabouts.

Regards,
-- Al

Did you try the cheater plugs? It is a common solution to hiss/hum problems? May not be elegant, but it does work in lots of cases. If it has no effect, you are out $10 at most. You probably need different amps, but the cheater plugs may buy you some time. If it really is a ground loop problem, different amps may or may not help.