I just have two questions at this point:
1. Why would you think you can spend X to build an amplifier but then sell it at Y? *
2. Second question, and this is the real kicker ... assuming you were somehow able to sell this thing at Y, what do you do when you've sold about 15 of them, you only have 5 left, and you still want to stay in business? **
* This is especially true when everyone else can probably spend less than X to build just one. They would have no prototyping costs, no case design and prototyping costs, no buying tubes at mass quantities and testing them (need to buy a few tube testers on ebay - that's fun!), no accounting for packaging costs, or especially: the point-of-sale investment and costs, etc., etc., etc. It gets too lengthy to list. It's sort of like buying and moving into a house. You may not realize it until after you move in, but your need-to-purchase "stuff" has just exponentially increased.
Further - if you buy standard parts from a typical retailer and assemble the amps yourself, just what is your business's value-added accomplishment? The fact that you built it? Can you make just your labor competitive in a world market? I seriously doubt it. There are a couple of individuals around here that work that way - one in particular that many of us know - but there's years of reputation that's he's achieved. Even then, the builds are mostly commissioned. They are also often based on teaming up with certain designers to restrict any possibility of free-lancers. That implies proprietary, unique designs. You've attempted to design this thing so far, but is it really unique - something that no one else has done? Where is your true added-value that's going to create the demand for anyone to buy it from you? IMHO, you need something more than simply creating a challenge in a single thread, thinking that you can design a tube amp, and create a demand simply because you've assembled it. You appear to have this uniqueness for some of your solid-state stuff, but sorry - so far I don't see it for tubes.
All of this dictates price and the resulting demand - it's everything between the success and failure of your "challenge."
** Simple - you have to re-invest or you're out of business. What happens to all of that profit margin then? For that matter, where did the investment for the original 20 come from in the first place? How many of us (even those of us already in business) can shell out $4000 for an experimental attempt at selling a design? Many people have trouble coming up with that much for a down-payment on a home. If you can't, you have to service that cost, too. And it all gets rolled into the FLOAT - the difference between your supposed profit and the need to keep things in stock in order to keep selling them. Put simply, your $6,000 profit quickly becomes only $2000 as you order another $4000 in stock - and it's all waiting to be assembled again. Now you have to find time to build inbetween packaging, shipping, and managing orders. Trouble is, if you are really trying to stay in business, that $4000 gets spent well before you have $6000 in profit - so if you're servicing the debt on the original $4K, you're already teetering on the edge. Meanwhile, someone decides to test the thing and tell you it's lousy (because it's tubes) or someone else starts a thread that says it shouldn't cost that much and dares anyone to prove him wrong.
Like I said, just a couple of questions.