Tube Amps are generally going to be just as reliable as their solid-state counterparts if you just observe the following things about them. Besides that tube amps kick butt.
1. Take a little extra care in transporting them. While you can generally bang around and lightly toss solid-state amps into trunks, back seats of cars, truck beds, etc., with tube amps you have to be a little more careful.
What I generally do when transporting tube amps is set aside a special place in the car so that the amp isn't going to be jostled around with hard shocks when i make abrupt movements with the car or hit bumps. Generally this means the back seat or the trunk with some towels or clothing around the amp. When you get to the gig, just make sure that you take special care not to bump the amp into other things, or to just drop the amp on the floor, if you're in the habit of doing that.
2. Make sure that there is proper ventilation for the tubes. Tube amps run hot--especially if they are wired in class A and they need proper ventilation. Generally this just means that you shouldn't put your polish cloth or set lists over the ventilation holes, and you shouldn't set the amp right against a wall so that there is no air to get to the vents. Some players even keep a fan on the amp, though i don't think this is necessary.
3. Change the power tubes regularly. "Regularly" means a lot of things to a lot of different people. GENERALLY SPEAKING, six months to a year is fine for most power tubes if you're gigging regularly (say, once a week or once every other week) and rehearsing regularly--and this means at typical club volumes. If all you do is play the amp at home at bedroom levels, the tubes can last two years or more. If your amp is bias-adjustable, then you need to make sure that the bias is set when you change power tubes. If your amp is fixed-bias, it's generally a good idea to stick with the same power tubes that came with your amp.
4. Make sure that a speaker is ALWAYS plugged into the amp. Tube amps need to see speakers plugged in at all times, so you cannot run a tube head or tube combo without the speaker plugged in! This is a big mistake and can cost you $200-300 if your amp blows a transformer. Even after you replace the tranny, the amp often sounds different--some describe the amp as being "neutered" after replacing a blown transformer.
5. Make sure that the speakers are plugged in at the proper impedance--or at the very least, a mismatch in the "safe" direction. The best scenario is to match the impedance of the speaker or cabinet with the amp--if your speaker cabinet is 8 ohms, your amp should be set for 8 ohms for best results. If you plug this same 8-ohm cabinet into the 4-ohm setting on your amp, that will also be safe, but you will generally lose about half of your power as a result of the mismatch. If you take this same 8-ohm cabinet and opt for the 16-ohm setting on your amp, you're going to likely blow the head up as it tries to produce twice as much power. again, this can result in a blown transformer and the same neutered tone if you have to replace it.
6. Use the standby switch when powering up and powering down. The standby switch allows the amp to sort of "warm up" and allows the tubes to settle into a sort of equilibrium state before it gets slammed with high voltage. If you generally let the amp warm up or down in standby for about 30 to 60 seconds when powering up or down, you'll find that the tubes last longer.
That's about it, really...taking care of a tube amp is really not as difficult as one might make it out to be. With a little care and some maintenance, the amp is going to sound fantastic for a lifetime!