Now if outdoors, even if on a small stage like at a state fair or Octoberfest or something, that's when you want the 12π hornsubs. Use enough for the SPL required. One or twenty-four, you'll get high-quality, deep, smooth output.

Back to the three π subs, they're each a 4Ω load and they'll each handle 400wrms. You could run one per channel or you could connect four in series/parallel and bridge the amp.

Don't skimp here, I'd use at least two for each of your mains and it wouldn't be out of the question to go for a ratio of 3-to-1 or even 4-to-1. Indoors, the more you have the better, and scatter them all around for best results.

That is exactly the opposite thing you would want to do outdoors or in a very large room. Outdoors, you want to group the subs together, close to the mains if you can. For coverage, most times they're grouped on each side but the point is that outdoors or in large spaces you want all sound sources within 1/4λ, or at least as close as possible.

Many people think this applies indoors too, but it doesn't. Indoors, standing waves and reflections from walls make interference that will be further than 1/4λ from the source. This self-interference causes deep notches at certain bass frequencies in various places throughout the room. By using several subs placed around the room, holes from self-interference from one sub will be filled by another. That's why it is good to spread them around spacially. It tends to average out the sound field and make bass response smooth and uniform throughout the room.

The moral of the story is to use as many subs as you can afford, and spread them around. Space them apart in all three dimensions, several feet apart. Put a few across the stage, put a couple up off the ground, put some at the front of the stage and some further back. Using several subs will help give you enough SPL in the bass range and it will also smooth the sound field. Where indoors bass is concerned, using the multi-sub concept, you can't have too many subs because you can always turn the power down. The more positions you have subs placed, the smoother and fuller it sounds.