Apart from the design side, the safety aspect needs looking at. Valve amps are full of dangerous voltages, satisfying your country's safety rules/laws will take some doing. Not to mention after sales support, marketing, taxes, accounts ad infinitum. You'll need a bizniss plan, cost analysis and all kinds of groovy things. Can't think of anything more likely to kill any enthusiasm for valve amplifiers than running a bizniss making and selling them. The days when Jim Marshall built his first amp are long gone; different world.
One other aspect occur's, namely Chinese imports; how you'd compete with those lads is anyone's guess. They are getting better at making decent low price valve guitar amps.
Another thing to be cogniscent of is the current trend for modelling amplifiers, which are used more and more in pro music.
Still if your young and full of beans have a go, do your research first though. Good luck.
Andy.
