I was going to suggest the Delta12LF that's used in the Theatre-3 and use that alignment. I found it to have tighter bass than the Alpha10 and allows for a smaller cabinet. However, the Alpha-10 is a much better choice if you're using a set of 2A3 amps. The "looseness" I found with the Alpha-10, when compared with the Delta12LF, may actually work in your favour when used with such a low-powered amp.A set of Wayne's corner horns would work well in your case too. However, already having the driver in hand is a pretty good reason.
Gar.