How about offering these as kits? Maybe even offering them
with or without tubes? If you're able to keep the cost low I
think you have a shot.

A few years ago, somebody (Ghostmoon?) offered kits to make an "All American
Five" five-tube AM radio. They were quite expensive, and the company did not
last.

Antique Electronic Supply (http://www.tubesandmore.com/ ) offers a few
tube-based kits for various things. You could try phoning them and ask what
sort of demand there is for that kind of thing in general.

I suppose you would be competing with existing solid-state kits. If you
Google around, you can find what's available and what it costs, etc.

Regards,

Phil Nelson