Here are my two cents. I have been building custom amps and offering kits since 2008 strictly as a hobby and right now I am at a point where I am dealing with backorders, which is okay since I pick my customers and they are pretty chill with my leadtime. The profits that I make supplement the income from my day job quite well, which allows me to boost my savings and to buy frivolous toys (tractors, cars, boats, amps, and so forth). Not sure if I could survive on tube amps alone and I am not sure if I want to since relying your livelihood on it would take the joy out of it. Again, I never ventured to make my tube amp building big, so who knows.
My recommendation is to keep your day job, keep building amps, establish your niche, and make some extra bucks. By the time you're ready to retire, you could then turn it into a lucrative business that could also be a fun hobby.
Min
www.tubeaudiolab.com