Two Rock problem: Crackling when PAB is on?

[heady dude]

Ive just got a two rock type 3 a couple weeks ago and recently, when it is on the PAB setting, there is a loud crackling distorted sound when you dig in to it, almost like static. It seems to only do it when the PAB setting is on. Also I hadnt noticed it before, until when I hooked up a T-rex roommate to the loop.
Any idea what this could be? Bad tube?

Thanks

[JoeB63]

try swapping tubes. If that doesn't work, send an email to Bill at Two Rock. He'll get you fixed up quickly. Excellent support from TR.

[heady dude]

Do you think its power or pre amp tubes? I just sent an email to Bill.
Thanks

[cracktop]

Preamp