To your tube questions...
1. Power Tube types make a huge difference. I have learned that I prefer El84's in all my amps. Its part of loving low wattage amps too though. A little google searching or lookin on the gear page will give you better info than I can provide. Also Gerald Webers or Aspen Pittmans books on amps are good sources.
Not only does each tube type differ but the ratings, manufacter, age, etc.. all figure in. There is a lot of boutique cork sniffing voodo involved here too. Your best bet is to try a few amps with el84's, el34's, 6l6/5881 and if you can find em 6v6's. Many amps can swap easily between el34 and 6l6 tubes, my Mesa Nomad has a switch for this...
2. Preamp tubes. More equals more gain... mostly. Sometimes different channels will use only certain tubes or add them together from channel 1 & 2. My Mesa Nomad uses progressivly more tubes from channel to channel. My Soldano Hot Rod 50 bypasses the first tube for the low gain channel. Also if there is a tube buffered effects loop it will use a tube, like my Soldano Hot Rod 50.
Again the number, type and manufacturer have implications here as well. For instance I put a 12at7 in the first position of my Soldano these days to lower its gain and smooth it out. Also a lot of voodoo involved here.
The most important thing to use here is some trial and error and your ears.
Where are you located maybe the members here can point you to some good shops. I find that in this arena the experince of the amp repair guy and the owner of Cowtown guitars make it worth not mail ordering. YMMV