Pre-amp tubes are the easiest place to start. look at your tube chart and start w/V1 - get a new one and swap it for whats there, if the issue is resolved then you are done, if not, go to the next one in line and repeat. If none of the pre-amp tubes seem to be the culprits, then try a new matched set of power tubes - the whole set. Make sure you note the tube type for the reverb driver, it may not be the same as the signal path preamp tubes. Process of elimination is a slow-but-steady way to see if the tubes are causing the noise.