Hey, so in response (I think):
1) Valve amps use tubes on two separate sections on the circuit. You usually have your EC types in the pre-amp section and the your EL/6L6/KT etc are in the power section. Most your EQ type tones will be through the pre amp section, and in truth, the number of valves makes no difference to the sound here, rather the design of the circuit will be the biggest influence. Your power section is where you get the saturated "gain" tones on these types of amps. Your power section valves is where you get "break" up when you drive the signal hard enough through the pre amp. Once again, the number of tubes will only be dependent on the design and has no real baring on the tone.
2) Tonal character is defined by the type and even manufacturer of the actual tubes themselves as well as the circuit design. Also, quality of other components makes a huge difference, although some manufacturers swear by cheaper parts as valve amps are not meant to be "hi-fi"
3) Power is determined by your power output transformers. Usually the 2 giant metal lumps or torroidal transformers that you see (and feel) in valve heads. These get bigger as the wattage increases as they are what is required to "step up" the wattage.