I had purchased a combo and was worried about the noise too... even had it checked out by Mesa repair center (Yorkville sound, who said everything was fine.) I was going to send it back to Mesa, but before I did that, I went with my combo and A/B it with other mesa (they didn't have a mark V) and it was slightly noiser but not too bad. Decided to wait until they got another Mark V in and go check that one out.
They finally got a head in. Went and tried it... it sounds exactly like mine does.
I think everyone's version of "quiet" is different. There's no way these are "quiet" as in 0 noise/hiss (try channel 1 tweed with effects loop in and at 12 o'clock, all Ch1 knobs around noon and turn the output up past about 11.. that is not "quiet")
Anyway, it is completely unnoticable in band situation. Can barely notice it in the store with just the customer noise... but at home in my music room with deadened walls and soundproofing (ie. it's dead quiet in there) it is sure noticable.