Absolutely no-one needs a 100 watt
tube amp today - the average pub/club sound system will have difficulty coping with anything bigger than the 18 watt Marshall mentioned above - especially if you are talking about a tube amp running flat out - as Jemplayer says, if you are starting out, look at one of the many 18 watt plexi clone kits available, or even better, a Trainwreck clone - if you don't know what a Trainwreck Amp is, and you're interested in amps, it's time to do some homework!!!!
At the other end of the scale, amps as complex as an Ecstasy or an RM100 have had many, many hundreds of hours of prototyping to get them to work properly - in high gain
tube amps it is often the final minutae of wiring loom placement, where the transformers are placed and all that sort of stuff which really makes them sing - and non of this is shown in a circuit diagram!