Is something wrong with my Epifani UL110?

[Chrisk-K]

When I attenuate the tweeter, the amount of hiss from the tweeter stays almost the same. For instance, there is little difference in the amount of hiss between 7 o'clock and 5 o'clock positions of the tweeter control knob.

[Marton]

It's the same on my UL-410. The tweeter will hiss much except if I close it, when I use it with a noisy head.