Re: I am now a man! VOX/Marshall
Rule of thumb that works for me - I get my gigging amps retubed and rebiased every year. General service gives it a once over at the same time.
Replacing pre-amp tubes is easy. Just swap them over. Not so easy to replace power tubes yourself. For most amps you will need to re-set the bias, otherwise it wil sound crap as the tubes will run too hot or too cold, which can have other issues besides just sounding crap...
The bigger issue is that unless you know what you are doing in a tube amp you can kill yourself - and I'm not kidding!!! The filter capacitors, even when the amp is off contain enough charge to kill you unless you discharge them properly. In your case I would book it into a competent tech and pay the money - well worth it. I also do that as a matter of course if I buy a second hand Marshall or other tube amp...
Different tubes sound different (cleaner, dirtier, etc etc). My advice is find out about different ratings of preamp and power amp tubes (12ax7, 12au7 etc etc and EL34, 6550 etc etc) and decide which best suit suit. Then check out the different makers of tubes (Svetlana, EH, EI, Sovtek, etc etc)