well keep in mind that you will need to get the amp rebiased, and as well probably recapped. electrolytic caps only last about 15 years usually, and this amp is far beyond that.
personaly for clean i would recoment jj/tesla tubes, they have a higher headroom, so they tend to stay clean a little longer. but i would say that a good quality groove tubes set would work well, like the 6L6R or the 6L6S (if you want a little bit more bite to the sound), or some svetlana(sp?) 6l6's. the electro harmonix 6l6GC's are nice too. preamp wise i will flat out say go with the electro harmonix 12ax7, it is by far the closest thing i have found to my telefunken 12ax7 stash of tubes, they are close but dont get dirty quite the same(not a bad thing though).
for the life of me, i can not remember whether those bassmans are tube rectified, if they are, buy the best quality tube you can afford, it makes a big diffrence.
if your not confused yet, im amazed.......
i would just get on
www.thetubestore.com and read the reviews, they can be quite informative. or if all else fails just buy a couple diffrent selections, and try em out.
PS if you dont like the sound of Sovtek tubes than dont buy the cheaper groove tubes, all they are is relabled sovteks. fender replacement tubes are the same thing. sometimes they even screen print their logo directly over sovteks.