Here, let me fill you in a bit about tube amps:
In a tube amp there's two different stages that contain tubes (aside from any rectifier, reverb, tremolo, etc. tubes - don't worry about those for now). The two stages are the preamp stage and the power stage. Both stages contain a certain number of tubes that will color your tone and contribute to the sound of the amp (in a good way). Preamp tubes are smaller in size than power tubes (that's good to know for future reference).
Distortion in a tube amp can be created a few different ways. The first way is via diode clipping - this is generally not considered a very good sounding way to get distortion from a tube amp. I'd avoid amps with diode clipping unless you have tested the amp and really like the sound. A more common way to get distortion is by overdriving the preamp section (the preamp tubes). This is how most Mesa and Marshall amps get their distortion - it's all coming from the preamp. A third way to get distortion in a tube amp is from the power tubes. This is different from preamp tube distortion because the amount of distortion is dependent upon how loud the amp is due to the positioning of the power tubes in the circuit. This leads me to wattage...
Wattage in a tube amp is different from wattage in a solid-stage amp. In a solid-state amp you will be able to achieve perfectly crystal clean tones at full volume. In a tube amp, however, the amount of volume you can have on your clean tone before the amp starts breaking up and distorting is dependent on the amp's wattage. If you want to have crystal clean tones without any sort of breakup at live band volumes, you're probably looking at an amp with at least 50 watts of power (depending on how efficient your speaker setup is).
As you can see, there are a LOT of variables in a tube amp, haha. Lots of things depend on other things which is why it is important to do your proper research on the amps you are looking at and make sure you can try them out in a store before you buy them.
Also, to clarify between a combo and a head: a combo is when the amp is all self-contained. The electronics and tubes are in the same box as the speaker(s). Like this:
http://cdn1.gbase.com/usercontent/gear/1605023/p1_u1nejyar5_so.jpg
A head and cab setup is when the electronics/tubes are in their own box and the speaker(s) are in another box. Like this:
http://www.marshallhalfstack.com/images/JTM45-TV.jpg
Or this:
http://vault66.org/stuff/amps/imgz/plexi2.jpg