If you can phase the sub perfectly to the mains you can get a lot more impact from even a small sub. This requires a sub with a continuous phase adjustment. (Not just 0 degrees or 180 degrees.) If your prepro can’t set a delay for the mains the best you can do is time align the sub 1 cycle late which is totally acceptable. Check: http://www.soundoctor.com/whitepapers/subs.htm
Play a pure tone at the crossover frequency with cable polarity reversed on main, dial phase on sub till you hear the null at listening position. Now perfectly out of phase. Switch to correct polarity on cable to main and now is perfectly in phase. Can’t overemphasize the importance of time alignment to SQ. Consider that a crossover or low pass filter at say 12 db per octave gives very large area of overlap. (40 Hz to 80 Hz is 1 octave. 60 Hz to 120 Hz is 1 octave) If the timing is out of alignment there is distortion of all the sound waves over a wide frequency range (partial cancellations and partial additions within each sound wave). Sounds like hell. Anyway, I have several systems going and several really good subs. Needed a sub for a 2 channel setup so I got a single 8" JL Audio Dominion D108, Just to add a little bottom to my "full range" Revel Performa F206s. Totally did the job in a 17’ x25’ room. Couldn’t believe it. Didn’t want to believe it (cause I wanted to justify a bigger sub purchase) Was not great until I phased it in however. Go ahead and scoff ye unbelievers! Went ahead an got a Revel Performa B110 sub anyway because it was prettier and the price was ridiculous. But can’t really say the resulting SQ is remarkably better, even though It does go a little lower. Set up properly the performance of a pair of JL Audio Dominions would beat any 10 sub that was set up improperly IMHO. :)
But them ain’t your grandpa’s 8" subwoofers.
Play a pure tone at the crossover frequency with cable polarity reversed on main, dial phase on sub till you hear the null at listening position. Now perfectly out of phase. Switch to correct polarity on cable to main and now is perfectly in phase. Can’t overemphasize the importance of time alignment to SQ. Consider that a crossover or low pass filter at say 12 db per octave gives very large area of overlap. (40 Hz to 80 Hz is 1 octave. 60 Hz to 120 Hz is 1 octave) If the timing is out of alignment there is distortion of all the sound waves over a wide frequency range (partial cancellations and partial additions within each sound wave). Sounds like hell. Anyway, I have several systems going and several really good subs. Needed a sub for a 2 channel setup so I got a single 8" JL Audio Dominion D108, Just to add a little bottom to my "full range" Revel Performa F206s. Totally did the job in a 17’ x25’ room. Couldn’t believe it. Didn’t want to believe it (cause I wanted to justify a bigger sub purchase) Was not great until I phased it in however. Go ahead and scoff ye unbelievers! Went ahead an got a Revel Performa B110 sub anyway because it was prettier and the price was ridiculous. But can’t really say the resulting SQ is remarkably better, even though It does go a little lower. Set up properly the performance of a pair of JL Audio Dominions would beat any 10 sub that was set up improperly IMHO. :)
But them ain’t your grandpa’s 8" subwoofers.