Well I should qualify that I don't consider myself an audiophile and that unlike the guys over on the Official Audyssey forum, I don't have room treatments, measured response graphs and the like. As we have very limited placement options in our living room, I'm sure that our room response is not ideal.
That being said, I have found the SB12 to match very well so far. Given the placement limitations and the need to match with the Orbs, I was looking for a sub that would not only fit, but had sufficient upper extension to work with the Orbs. Since the SVS site had a frequency response chart that cut off at 100hz but they claimed response up to around 200hz, I asked for a full range plot, which they sent me. The full plot showed that the sub was down by about 1db at 120hz, 3db at 170hz and only about 5db at 200hz. Since my L & R speakers are set by Audyssey calibration at 120hz, the sub covers that range on down very well. My center is set at 200hz due to an idiosyncracy of how it's mounted, but given that the center is mainly dialogue, the lower level from the sub at this frequency is not much of a problem.
I ran Audyssey about 2 weeks ago and have noticed that things sound subtly different now which may be due to "break in" of the driver. I'll probably re-run Audyssey later this week.
I am very happy with how the sub integrates with the Orbs. I have been listening to a lot of 2 channel music (L & R only) and the transition between the sub and the Orbs is very smooth.
Overall the SB12 has achieved what I wanted. I no longer have "port chuff" on strong/deep movie tracks, the lower end is much deeper in a practical way and bass lines in music are much more articulate and textured. We don't listen at huge levels, but I'm sure the SB12 could achieve pretty loud levels in our room (odd shape about 3000 cu ft.).