I've run these things both ways, along the narrow wall and along the wide wall. As you might expect, it's usually most pleasant when the room depth/width ratio is somewhere between 1.5-to-1 and 1.7-to-1. But I've also had them in rooms that were more than twice as long as they were wide. The "sweet spot" is a pretty wide area, really.Generally, in the situation where a room is much longer than it is wide, I like to have the speakers installed in the corners at each end of a short wall, and then the preferred listening areas are in the half to two-thirds of the room nearest the speakers. But they can be installed in the far corners too, putting the best listening area roughly centered and back near the opposing wall.
Honestly, being 20 feet apart doesn't concern me. That's no problem at all. But how far back are we talking about here? You mention that you'd be 12 to 15 feet back, so I would assume that your room must extend deeper than 15 feet. If that's the case, you're really not in too bad shape.
I'd go with the seven π-18, just as you've suggested. You can use a JBL 2241H or 2245H, or you can use a Magnum 18LF. The cabinet is basically the same for any of these, being 7.5ft3 and tuned to 30Hz.
You can also use the JBL 2242 or Eminence Kilomax 18. The Omega 18 or Magnum 18HO can be used as well, and they generate an EBS alignment when used in this cabinet. It works very well with the corner gain, but using a slightly larger duct that tunes to 40Hz is also an option with these two drivers. That will give a little bit flatter response at the expense of slightly reduced extension.
All of these are suitable drivers and you can expect performance to be quite good. I would suggest using a 5mH coil in series with the woofer, and the mid/hi crossover as described in the π midrange horn implementation post.
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