Most pop recordings are mixed mono below 80 or 50 Hz. There's literally a switch for it on most mixing consoles. For minimally mic'd classical, that's not the case though. True stereo subs will give you an amazing increase in soundstage, low frequencies being one of the psychoacoustic triggers to clue our brains to venue/room size of the original space. Once you've heard that difference on a recording with true stereo bass you'll be an ardent believer! Not many recordings have stereo bass but when they do, oh baby!
The Behringer and other solutions are all valid so I'll let you compare the validity of those and suggest another possibility: Purchase the passive VMPS subs. Get a quality sub amp. Parts Express and many others offer them. Parts Express uses licensed technology "tracking downconverter" from Sunfire for $300 or $400. Get the dspeaker sub correction device from simplifiaudio.com Highly reviewed in The Absolute Sound and many other places. Only $400 so you could get one for each sub if you have the budget. Corrects for time, phase, frequency, much more than even a parametric eq does, similar the Audyssey suggested by another, but only in the low frequencies and for much lower cost. You're not going to sacrifice any of your HDtracks 24/192 range at low frequencies with this device. When you're able to place the subs in the corners and then allow this to correct for time, phase, and frequency, the nulls become much less of a problem, especially at your prime listening position. You will essentially have near perfect bass response. Just adjust the crossover on the sub amp until there is no overlap. I'd start at 30 Hz with your speakers and inch up gradually until you have the right blend.
The Behringer and other solutions are all valid so I'll let you compare the validity of those and suggest another possibility: Purchase the passive VMPS subs. Get a quality sub amp. Parts Express and many others offer them. Parts Express uses licensed technology "tracking downconverter" from Sunfire for $300 or $400. Get the dspeaker sub correction device from simplifiaudio.com Highly reviewed in The Absolute Sound and many other places. Only $400 so you could get one for each sub if you have the budget. Corrects for time, phase, frequency, much more than even a parametric eq does, similar the Audyssey suggested by another, but only in the low frequencies and for much lower cost. You're not going to sacrifice any of your HDtracks 24/192 range at low frequencies with this device. When you're able to place the subs in the corners and then allow this to correct for time, phase, and frequency, the nulls become much less of a problem, especially at your prime listening position. You will essentially have near perfect bass response. Just adjust the crossover on the sub amp until there is no overlap. I'd start at 30 Hz with your speakers and inch up gradually until you have the right blend.