Help with nait 2

My nait 2 is acting up. On anything but the lowest volume setting, the tweeters are making sounds as if they are blow, but they of course are not. Everything below the treble is fine. But the treble comes out all distorted (and, like I said) as if the tweeters are blown.

Can this be a capacitor problem, or do you suspect something else. I have traced the problem to the amplifier section. When used only as a pre-amp, it sounds fine.

Judd