I'll share my thinking in case it helps you. I'm contemplating the same upgrade - going from 1 Kinergetics sub to using a matched pair of subs. I've read other posts here about this subject and Mdhoover is right, the thread he refers to is hilarious but does have some useful information.
It seems from the posts that the people who do use 2 subs swear by it and the people who don't seem to be of 3 opinions: 1. those that aren't sure of the benefits of stereo subs, 2. those that think it's unnecessary (1 should suffice in most rooms), and 3. those that think it's detrimental (cancellations, etc.). I'm currently in the "1" camp but am willing to experiment and keep an open mind. I know I want to get atleast 1 good hi-end musical sub such as REL, James or the Martin Logan Depth but am considering trying a set (ACI, Vandersteen 2QWs).
The 2QWs have 3 8"down firing drivers in each of their sealed cabinet subwoofers (I've heard sealed subs are better for music vs the ported designs). But the 2QWs use a line level crossover between the preamp and amp and requires you to connect the subwoofers via the speaker outputs of your main amp (you add separate speaker wire connections on top of your current front channel speaker connections and their system somehow knows how to direct the low frequencies, based on adjustable crossover settings, to the subwoofers' internal amps while still passing on the signals to your right and left speakers). This concerns me a bit but Vandersteen says it doesn't change the impedance your amp detects, although they do need to know the input impedance of your amp (to make sure their system will work with your amp?). And I'm not sure if the low frequencies directed to the subwoofers are also sent to the left and right speakers but I think they're not.
What I do know for sure is that, IMHO, a good quality subwoofer adds quite a bit to the listening experience by adding the lower octaves which increases the presence, tonal balance and realism of music. Hope this helps a bit.
It seems from the posts that the people who do use 2 subs swear by it and the people who don't seem to be of 3 opinions: 1. those that aren't sure of the benefits of stereo subs, 2. those that think it's unnecessary (1 should suffice in most rooms), and 3. those that think it's detrimental (cancellations, etc.). I'm currently in the "1" camp but am willing to experiment and keep an open mind. I know I want to get atleast 1 good hi-end musical sub such as REL, James or the Martin Logan Depth but am considering trying a set (ACI, Vandersteen 2QWs).
The 2QWs have 3 8"down firing drivers in each of their sealed cabinet subwoofers (I've heard sealed subs are better for music vs the ported designs). But the 2QWs use a line level crossover between the preamp and amp and requires you to connect the subwoofers via the speaker outputs of your main amp (you add separate speaker wire connections on top of your current front channel speaker connections and their system somehow knows how to direct the low frequencies, based on adjustable crossover settings, to the subwoofers' internal amps while still passing on the signals to your right and left speakers). This concerns me a bit but Vandersteen says it doesn't change the impedance your amp detects, although they do need to know the input impedance of your amp (to make sure their system will work with your amp?). And I'm not sure if the low frequencies directed to the subwoofers are also sent to the left and right speakers but I think they're not.
What I do know for sure is that, IMHO, a good quality subwoofer adds quite a bit to the listening experience by adding the lower octaves which increases the presence, tonal balance and realism of music. Hope this helps a bit.