I'll talk to the owner when he is ready to do the sound system and see if he wants to go with the 4520 or the version posted here that uses a single 18" driver.
I suppose some are using the 4520 as bass and the long throw cabinets as midbass perhaps.
Its nice that people have taken a design and improved on it. Shows that it was a good basic design to begin with.
I do agree the 4520 isn't a subwoofer.
I like how it sounds though. Brings sort of a realism to what bass it does produce and the bass sounds so natural and effortless probably due to less distortion and such because the speaker cones barely move while producing loud bass levels.
The other local rink has four and they sound ok.
One of theirs at one point was loaded with two drivers labeled concert on the dust caps. Probbaly car audio subs. That one would go deep far as bass response was concerned, but it didn't necessarily sound as natural as the other cabinets.
So here's what I'm thinking.
1. 4 of the 4520 or 8 of the 18" version for the bass
2. 4 bullfrog or Altec 816 cabinets or 8 of the same for 90-800Hz
3. Horns for the treble 800-16KHz
4. New Yamaha amps with built in DSP.
5. Either a passive crossover on the midbass and treble horns or double the amps and the built in active crossovers.
EDIT:
Now if I build the 18" version of the 4520 what frequency should the speaker be crossed over at?
Also what driver should I use since the PD-1850 is not available in the USA?