My latest creations are up and running, here's one:
The woofers flanking the 511 are SEAS W18E001s, crossed at 800Hz. The 511 is using a TAD TD-2002 driver. The 15" in the module holding everything up is a Dayton RSS390HF-4, crossed at 60Hz, and EQd flat down to 20Hz.
The idea is that the MTM arrangement more or less eliminates the 511 pattern flip; the vertical dispersion narrows to about 50 degrees at 500Hz and stays near there to about 8kHz, where the 511 beamwidth starts to fall.
Francis