All about Tubes:
How do you know when an amp needs new tubes?
How do tubes change the sound?
What tubes for what sounds?
All types of tubes made by different manufactures some sound different then others. It can be pretty expensive to try a test of different tubes in an amplifier. It's probably best to go with something you may know. Also, your amp was designed with a certain type tube set in mind and I usually tell most people to re tube their amp with the spec tubes for the amp. If you play hard or are a professional musician, you should replace your tubes every 6 months to a year in order to retain the full power needed to play in a live or club setting.
If you just play for fun then you need to change tubes every few years unless you have a bad tube; of course that would be replaced as needed - It's really a no brainer.
Bench Rates start at about $105 for a flat fee repair plus parts. Bias test / set up with new tubes usually costs about $80 takes from one to three days to turn around.
Tubes do need to get replaced every so many hours of use (Eddie Van Halen used to replace his tubes every night, but that's pretty ridiculous). It depends on how often you play and how hard you drive your amp. If you play every day, they should be replaced every 6 months to a year. Sovtek & Electro Hamonix tubes are good, Telefunkens can cost over $1000 each. What you may not be aware of is that a tube amp will never sound good unless it is driven near or at peak. That's why it's hard to get a great sound out of a 100 watt Marshall because your band mates will be screaming for mercy before you get any great tone. So, crank your amp all the way up and see how it sounds, there are testers for tubes, but your ear is the best gauge, remember, any tube amp will sound like crap at low volume.
You can tell if tubes need changing by their color and tone. When the amp is on take a look at the back, if they are not burning really bright, or there is a blue-ish glow you could probably use a new set. Tone will also tend to be dull... high/treble will not be as high and the bass/low will be a little murky. I have a Vox Ac50 - I prefer JJEL34 power tubes and TungSol 12ax7 preamp tubes. What tubes to use for what sounds is a difficult question because everyone has their own preference.
It rates tubes, explains their tonal differences and really gets in depth on what you can expect from each type of tube. Remember to take your amp to a certified amp tech for re-tubing... it will have to be re-biased and you can electrocute yourself and blow out the amp in the process.