Your experience of moving to two subs

good rule of thumb on this subject

if you are listening to music (leave aside home theater movies booms and rumbles) and can hear and localize your subs, you are doing it wrong

the bass should just be in the room, coming at you like waves of energy

Update with a few notes on my short journey here:
(@soix, tagging u since u asked about my findings)

1) Reconfirmed more thoroughly my claim about aural locatability of my Rythmik F12G sub. Anyone can believe me or not, but when it’s only the sub playing music with a very low crossover setting, I can instantly locate where the sound is coming from in the room while facing any direction and standing almost anywhere in the listening room... except when I’m behind the vertical plane made by the sub’s cone. Then it’s a diff story and it’s not locatable. My ears have to be in front of the driver somewhere.
2) I had placed the single sub in the middle of my speakers because I needed the SPL from that single, relatively low output Rythmik, which is focused on speed, not loudness. So that position was necessary (you could say optimal... or not) because I really needed two of these subs to begin with in a room of this size, but only had one. AND, the extra low crossover point sounded the best in that position because of the directional effects I’m hearing with this single Rythmik sub, and how having it pointing right at the listening position affected the sound in the 40+ hz bass range.
3) Having the two subs in play (Rythmik F12G and SVS SB2000) far apart from each other totally alleviated both the directionality/locatability problem, and the issue with needing to use an unusually low crossover point.
4) The Rythmik is a totally different animal. I listened to both of them with speakers off while doing test tones for phase adjustment and with music. There’s actually music and details coming through the Rythmik, both in the lowest frequencies, and up through top of the audible range as it diminishes through the end of the crossover slope. The SVS is just blub, blub, blub, blub, and you really can’t make out musical details anywhere in its output. In comparison, it’s a bunch of noise and sounds really distorted. The Rythmik has so much more purity in its sound. I wasn’t able to get the SVS to blend in with my speakers, blubbing along like that. The Rythmik just disappears.
5) I bought another Rythmik. :)

Ther seems to be an assumption that bass is omni-directional.  It is not.  There is a point of origin for the original signal and two subs can resolve that reality.  It is like two ears helping locate the direction of a sound or two eyes resolving depth perception.  The only instance I can think of that might give false clues as to imaging is if the bass is recorded using a DI [direct in] input to the recording soundboard.  In that case, the bass is wherever the recording engineer decides he wants it to be.  "Prndlus" seems to be closest to this reality when he discusses soundscape and says, "And we want every directional cue we can get for the best soundscape ".    I agree !

I've got two subs run off miniDSP.  I think some kind of DSP is almost essential with subs. I am also running the subs left and right channels. At that point, the subs completely disappear. What I really hate is being able to locate the subs in space. 

If I run the subs by themselves, they're really not loud. but if you run the mains without the subs it's very clear that the music is lacking.  That, to me, means something is right. 

It's worth doing some kind of room eq and dsp to tune in the subs, and while one sub is good two, run left and right channels is what works for us. 

Thanks so much for the update. As a fan of Rhythmik subs and an owner of an SB2000 (for home theater only) I especially appreciated your observations comparing the two subs — most enlightening! Glad to hear you found benefits in using two subs, and I’d love to hear your thoughts once you get the second Rhythmic set up and dialed in. I have no doubt it’s gonna be great and color me a little jealous, and there will be a pair of Rhythmik subs in my 2-channel rig as well at some point.