For Ampeg, how about the BSE-410HLF? It goes a bit lower than the 410H and it has a higher sensitivity -- on the downside, I think it weighs about 15 lbs more.
Of course, you could always try to find an old matching Sunn 2x15! The "new" Sunn (Fender) 4x10 and 2x15 are also something to consider. You might be able to pick one of these up cheap as music stores may still be liquidating them.
Good luck!