i think some things needs to be clarified again, ie 25-30w is not alot louder than 5w. In fact, 15w is also not alot softer than 100w. Wattage doesnt tell u much about volume, but head room (How clean your amp can sound until they break up).
On the notion that u have to crank ur tube amp than it sounds gd, its also not necessarily true. Some amps are built to stay clean no matter how u crank them, the some ppl dislike how the ac30 break up for example (The are a few different ac30 models), and of course ur marshalls which sounds crappy until u crank the 100w monsters till the dirt comes through (This would be too loud even to play in a small club venue). So its all about what the amps are made to do. You do not have to crank them at 80% then it will sound nice, u just need a gd amount of signal to be fed and the power tubes to be working hot enough. Of course, if u can play louder by all means. Also, i find that at higher volume, ur eq is going to react n sound differently as well.
The magic is actually in the cone imo, u need to play at a level where the cones open up and come alive, when too soft its like muffled and as tho there is a blanket over it. So whether tube or not, u need to understand that about cones.
If u are getting a head and cab combo, then seriously look at the speakers combination with ur amp. Thats gg to make more dif than changing ur tube to a certain brand from stock ...etc.
Honestly, i owned a few tube amps alr and i dont find that the maintenance is higher in anyway. Tubes are more solid than u think and can last a gd deal. Most of the cost comes from us having itchy backside n keep changing tubes to try get better tone.