I have a friend who taught himself amp building and electronics and he has built some fine gear, he moved out of the area years ago but looking inside the units there is not a lot to them but obviously they are built to high specs, but going back to my buddy, he built a valve guitar amp and it was a work of art, he used high quality components and all the wiring was point to point, as neat as the pics and then he made a wooden casing to surround the metal chassis, it was only 50W but a very loud 50W with a matching cab which was balanced to the output and ported.
He said the cost was very low but you have to remember his own personal time.
I would like to have an attempt at that, maybe start off with just a 'ready to build' kit and see how I go. I have repaired amps in the past but just minor stuff(ripped out power cords, smoothing caps, broken pots etc) but never built one. A retirement project maybe??