Here's my sub + SSV experience:
I'll just say up front that what follows is opinion. Feel free to disagree. I included more detail since these are subjective assertions.
I find the low end lacking with the SSV alone. I'm not bothered by absence of thump or lack of floor shaking. The Kronos as a sound source has a broad spectrum (full low end) and when using the SSV alone I miss the low end on things like organ, EP, bass, low octave piano.
I decided I wanted to add a sub for small venue/practice. I didn't want a massive rig and wanted to keep weight down. I dont need this sub to supply the spl for medium or large venues.
Based on all of the positive experience and good reviews, I ordered a Behringer B1200. I routed my mixer out to the Behringer and experimented with the cutoff frequency (or not). I listened to various familiar music sources and also played the Kronos/PC3A6. To put my listening impressions in to perspective: The listening area comparison was a pair of Yamaha HS8 tops and an HS8 sub. I realize that studio monitors are not a fair comparison. For medium or larger venues I am accustomed to the sound of multiple 18 subs (again, not the goal here).
I was pretty disappointed with the overall sound of the SSV+Behringer B1200. I spent about 4 hours tweaking and troubleshooting. I found the Behringer sloppy and lacking full character for bass representation. The Kronos Berlin piano sounded empty and muddy in the low end. The Kronos bass sounds were lacking depth and also poorly defined. I listened to the Behringer alone and it was a mess to my ears (I'm familiar with the sound of a sub alone). To my ears, the Behringer didn't extend as low as I'd like. The Behringer did add bass to the overall sound and did get loud enough for my needs, I just found it to be non-musical and inaccurate.
I went in search of an alternative. I was worried I'd have to get into the 75#+ size to be satisfied with the sound. Today I headed out to audition subs with a target of the Electro-Voice ZXA-1 sub. Briefly, the EV ZXA-1 sub retails for 599 USD, uses a 12" woofer, 700w amp, and weighs 46#. Its size is similar to the Behringer b1200. I listened to the ZXA-1 sub in store and compared it to the QSC w181 amongst others. The zxa actually compared favorably to the bigger subs. The sound representation was quite similar. Pushing the spl revealed the differences between the larger more expensive subs and the EV ZXA-1, which was not my main concern. My in store audition impression was that the musical representation by the ZXA-1 was satisfactory. The compromise compared to more expensive and larger subs would likely be spl if used for venues with many bodies. So I headed home with the EV zxa.
Back in the same environment with the same sound sources, I was now happy with the sound. The only tweaking I did was adjust the sub volume to taste. I ran stereo XLR into the EV zxa-1 sub and stereo XLR>TS out to the SSV3. The Kronos sounds awesome with the SSV and EV ZXA-1 sub. The piano is full and clear in the low end. The bass and organs are well represented. The EV sub gets quite loud, FWIW. I really like the combo because it remains musical going from soft to as loud.
It isn't fair to compare the $299 Behringer with the $599 EV, but for me that is where I needed to be at entry. I couldnt work with the Behringer wish I could. YMMwillV.
Here is a pic of the EV ZXA-1 sub + SSV for scale:
http://i62.tinypic.com/eqns7o.jpg