Tube Amp Technical Question

I have a pair of Wright Sound Mono 10 amps. Recently, I heard some static out of one of the amps, and now, although I don't hear static anymore, one of them produces significantly lower output than the other (about 5db measured with a sound level meter). I tried swapping all the tubes between the two amps, but it made no difference; the amp with the reduced output before the swap still has the reduced output after the swap. Anyone have any idea what could be wrong?