Advances in tube circuit design in the last 30 years are almost non-existent; after all the SE triodes date from the '30s. So every competent technician can copy an old design, use high quality modern parts and come out with a decent tube amp. Many of the minor manufacturers are doing just that. The only difficult part of the design are output transformers, and the good ones are terribly expensive. So you can really do a fine DIY tube amp, if you do not electrocute yourself with the high voltages ;-) Making a great tube amp though, is extremely difficult.
