Here's an idea for a medium efficiency speaker.
Lambda TD15X in 5 cuft enclosure with a slot port tuned to 35hz
Lambda TD6.5M in sealed enclosure target Q of 0.5
RAAL 150
XO's in the ballpark of 300hz (2nd order) and 2.5khz and I'd like biamp with the TD15X getting a beefy amp and MT getting a sweet tube amp. Behr CX300 would do the trick for the active xo.
Depending on how much baffle step you need for your room placement, you can acheive an honest to goodness 93-94db from 35hz all the way up. Not SPL limited over bandwidth like other so called hi-efficiency speakers.
Not a cheap speaker and not one I am building....I have plans enough, but I just thought I'd throw it out there to chew on.
Another idea is to put the TD15X in a smaller sealed box Q ~.5 and add subs for first octave.
I'd put this up against anything commercial under $10K. The well behaved cone behavior of the drivers well out of intended passband would make xo pretty much a cinch.
