Well, working in a similar field, streamlining your production is the whole game. Things like having a standardized chassis, instead of having to drill out a custom chassis every time you build, will go a long way. If I was you I would look for things you can "production line." For instance, if you had that standardized chassis (you don't need to use every hole on every amp, of course), you could make a run of, say, 12 of them at the same time. This saves you a lot of set up time. Similarly, you could easily production line the building of cabinets. Depending on how similar your circuits are, you could probably do the same thing with your circuit/tag/turret boards (whatever you use).
The other really helpful thing, production wise, is to build in batches. You do the same task on each amp of a batch, and again you are saving on setup time. This of course requires that you make the up-front investment for parts in (say) 12 amps all at once, but then you can probably get bulk discounts when you buy like that. I don't know who you are buying your transformers from, but I'd be willing to bet they give bulk discounts. And given the price of iron, that discount could be the difference of a couple dollars an hour.
Also, are you a registered business? If so, you can probably get a wholesale account with at least some of your suppliers. The standard discount for most wholesale account is 50%, so it makes a big difference.
But of course, it really all comes down to the old joke - a luthier wins a million dollars in the lottery. A reporter asks him what he is going to do with the money, and he says, "well, I've got some bills I'm going to pay off."
The reporter asks, "what are you going to do with the rest?"
"Oh, they're just going to have to wait."
Gabriel