I have a series 2 rectoverb and since I cannot always crank it I use my MXR EQ in the loop to cut the volume a bit so I can crank the master on channel 2 (usually vintage). As soon as I get the master above 12:00 it starts to break up in a bad way, i sort of cuts in and out. Could this just be a bad preamp tube, possibley in a later gain stage. Channel 1 does not have this problem even if I max out the master. If it is a bad preamp tube I'm not sure which one to replace. Any ideas?