I found online that the minimum ported box volume for the EV SP-15 is 4-8 CU FT.  I am below that with the standard 1950s design.  I will cut out a 12" X 12" door between the two chambers.  I sure that will help a lot.  Cheers.

In my simplistic view, the chamber behind the driver and before the throat needs to compress air in order for the horn to work.  If the Vas of the driver isn't a lot bigger than the chamber, there won't be any compression, and it won't work like a horn (though it might work some other way).

In my setup, the Vas of the driver (Altec 515E) is MANY times greater than the compression chamber (1.1 cu ft.), and I get SPL of 115-118 db in the horn mouth with just a few watts.

I wonder if the ratio of Vas to chamber size is meaningful?  Seems like it would be...

Thanks, Steve!  In my case, the Vas of the Altec 515 (23.5 cu ft) is literally 20 TIMES the chamber size (~1.1 cu ft if I'm measuring the right space).  If I'm not including enough area behind the driver, it's still got to be > 10:1, Vas to chamber.  This seems to be working well, but I suspect it's got more bumps in the FR than if chamber and throat were optimized.  I wish there were some rules of thumb to guide me.