Originally Posted by Rex Anderson
One line of thinking is to buy main speakers with flat response down to 80Hz and use two (or four) subs for 80 Hz and below. As others have said, using well placed subs in your room yields smoother LF response to more seats.
Only thing I'd say is that flat down (only) to 80hz may not be ideal unless you're using steep crossover slopes. If you use the typical 12dB/octave slopes found on most receivers, then your mains will still be asked to produce a decent amount of 40hz content. If they can't, then the x-over response will not be ideal. How that impacts your in-room response is unknown without measuring, of course.
Thus, I prefer my mains play flat down to at least an octave below the chosen crossover point. IMO, this helps blend the mains to the sub. Since I tend to land on a 60hz x-over point, I tend to always buy full-range mains that can play with authority down to 30hz.
P.S. It also helps to have subs that can play cleanly at least a half-octave above the chosen x-over point. Since most AVRs have 24dB/octave slopes on the low-pass, the subs still get (with 80hz x-over) up to 120hz at 12dB down, and if they muddy that then they ruin the blend to the mains, too. 120hz is also directional, so if your subs are boosted (house curve) you may find them calling attention to themselves with an 80hz x-over point.
edit: I just thought of something else -- there is a little...nit...that the receivers tend to use 12dB slopes on the HP and 24dB slopes on the LP, which to "sum correctly" would want the mains to roll-off another 12dB. I suppose this is one argument for using speakers with an 80hz knee...but, I'm still not a fan, lol. I suppose this is because I prefer a naturally tilted response and having some extra energy down low from the mains makes me happier, lol.