Here's an idea using what he's got. Put the JBL 12s up on sticks and the Yamaha's on the floor as aux fed subs with an elemental analog crossover and hopefully some sort of HP. The horns should be left in as the hole they'd leave would affect the cabinet tuning. The Yammies aren't going to produce any real bass but they will add a bit of thump to the kick drum at 80-100 that wouldn't be there with speakers up in the air. By not asking the 12s to produce any bottom, they should have a bit more clarity. That is about as loud as this pile-o-boxes will go. Any louder would be a complete modern system up to the task.
Probably the forum members have been over-thinking this. If so, this is the best advise yet! If the system is just going to be used at a party with a hundred people of questionable sobriety that want some music, this will be adequate and appreciated as much as a modern "state of the art" active full range with subs system. My experience is that 98% of the people just need it louder than their home stereo, want a little bit of kick and bass they can feel in the gut, and if they can hear the vocals, they are happy.
If a real quality system that can compete with the "state of the art" is the poster's desire, that just can't and won't happen.
