However much I liked the Jan/GE 5751 in v4, it really reduced the noise I was getting with the reverb with it installed in v6. I did try other tubes in the PI position (tubes that are known to be quiet) did not make any difference. I pulled the cables out and still had the noise but not as dramatic. So far the only tube I have not replaces is V3. None the less, with a lower gain tube in V6 reduces the hum considerably. Reverb sounds really good now.
Due to the design of the amp circuitry and considering how sensitive it is, the low level hum is tolerable. As for the other Mesa Amps I have, the Mark V is extremely quiet, audible transformer hum is difficult to notice unless you turn up the volume and gain to stupid levels. Mark IV, not too bad either, but will make similar noise as heard in the RA100.
Noise in the RA does not seem to be channel dependent. I will have to get a replacement tube for V3 and see if anything changes. I may just swap it out with a "known to be quiet" tube and see if there is any difference.
Even with the hum, it does not stop me from enjoying the amp.