Also check out the Kendrick Amplifiers website. Gerald Weber has been building boutique amps (and servicing all amp brands for years). His instructional books and DVDs are FANTASTIC, and you can learn everything there is to know about tube amps.
Here is the link to the books and DVDs on their website:
http://www.kendrick-amplifiers.com/mm5/merchant.mvc?Screen=CTGY&Store_Code=KOS&Category_Code=054(Disclaimer: I have absolutely no affiliation with Kendrick; I post this link as a customer who thinks they do a fantastic job and provide really great customer service). You could probably find these titles on Amazon, Ebay, etc...
Kendrick serviced my '76 silver face Twin Reverb years ago, and they did a great job.
Other thoughts on your tube question:
Do some google searches on mods for the modern fender amps. You'll find all kinds of suggestions. In particular, I know some people have found that replacing the filter caps and a few other key components go a long way to "improving" the sound of these amps (although IMHO sound quite good out of the box).
Anyway, welcome to the rabbit hole that is the world of tube amps!