Just to comment on this as I just picked up a used TA-30. The hiss is quite noticeable on this one, both channels. Worse on the 30 watt setting, not as bad on the other two for some reason. I tried turning the reverb tank, made no difference at all. Tried just moving the cable around with a pencil, I could hear it having some effect, seemed to get quieter the further from the board it got. I didn't bother unsealing though, didn't look like there was much length to play with. I also tried 3 different tubes in V5 and V6 but the Mesa's in there were the quietest of the bunch.
What did work for me was the cut control on the green channel. With the amp in the 30 watt mode dialing the cut up to about 3 o'clock on the dial eliminates most of the noise. I can actually hear it cutting out, strange. I then dial some top end back in with the treble knob and I'm good to go. Seems to work in both normal and top boost modes. It's a pretty bright amp anyway so dialing in that amount of cut is not a problem for me.
edit - pulling the cut actually stops all the noise completely. I wish I could do the same on the red channel