This whole stereo sub thing is nonsense. Hardly any info there. Hardly heard. If you can detect a difference, it's because there are now TWO subs in the room instead of one, and the strain on the single sub will now be halved. It's the transients you hear first when it comes to bass, and those transients are almost always located above the crossover point..ie..in your mains. The ONLY possible way you "might" be able to increase the "spaciousness" in your subs is if you physically locate them directly to the sides of your head (ala Lexicon pre-pros)...and even THAT is of dubious benefit. MUCH research has been done on this. If you really want to improve the bass, yeah, buy two subs...but DON'T bother running them in stereo. Either stack 'em or spend a LOT of time and effort situating them properly.
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