I prefer using a minimum of two. I have had as many as four 8" drivers fashioned in individual closed boxes. That said I don't have a set preference on hook up methods I find it can depend on system/s. Sometimes the HLs make things easier to set up and also sound better in the end. And versa-visa. I have had a bunch of subs and like a bunch. Also look into Rythmik. I'm am currently awaiting a venture into two "home made" Rythmik servos ( a sprained forefinger can be a bigger deal than it would otherwise seem). I convinced a friend to go their way and buy two of their sealed subs, glad I did. More importantly so is he. And a + 1 for Seaton.
Should I get a second sub or a higher quality single sub?
I have 600W mono-block amps and excellent speakers. I recently purchased a JL Audio e112 sub. The e112 blends seamlessly with the speakers giving a nice big tone and not too much bottom. Actually, I never really "hear" the sub, it just seems to extend the speakers lower frequency range. The only problem is that when playing the system at moderately loud levels the volume of the e112 must be turned up to the max. I know the sub is operating properly because I returned it to the dealer to be checked out, so I am not sure exactly why that happens.
So, I have been considering whether I should purchase a second e112 sub, or sell it and upgrade to a JL Audio Fathom 112 (the Fathom series of subs is better than the e series). Does anyone have any advice whether I should get a second sub (an e112) or get a better, more expensive single sub (the Fathom 112)? My listening room is 25' long X 12.5' wide X 7' high.