I was going down this path at one point, but decided against it. Subwoofer integration is *very* difficult from talking to two people I've spoken to.
A friend of mine has the TacT 2.2 and he's integrated a Rel into his Sonus Faber Signum setup. The TacT will give you significant control for the integration. He's happy with his integration, but he's come a long way from just simply adding a subwoofer.
Another friend I know actually builds speakers and is *very* technically saavy with designing his own crossevers. After a very long and technical discussion, I came away with the concept of woofer speed differences; in order to come close to a 'good integration' the woofers of both speaker systems (the satellite and sub) should be within range of each other. For example, I was discussing my setup and what would be a good starting point - I have a D'appolito layout for my main speakers with 2 6" drivers for the midbass. To get a proper subwoofer to match the speed of my main speakers, I would want to go with the same size drivers for the subs. Which lead me down to the VBT line of subs. I remember going to the HE2004 show this past summer to hear the VBT, and I did not like what I heard, but then again, it was a hotel room where acoustics are terrible.
I was out in a high end store in the Bay Area and my girlfirend and I listened to a pair of Avalon speakers (can't remember the model, but it was their top end with diamond tweeters) and the system had the top end Rel sub in the system. The bass was not coherent at all, and this was a fully optimized system for the listening room. Both my girlfriend and I thought the system sounded much much better with the sub turned off. The sales guy thought we were crazy, but keep in mind, they just want to sell you gear.
I guess what I'm trying to say is, you're probably better off upgrading to a speaker with more range than adding a sub to a pair of bookshelf speakers. From what I see, you've invested a significant amount into your system. Why not do it the right way and look for a floor stander rather than add a sub to a pair of bookshelf speakers?
But if you're sold on going the sub path, Richards above has the right idea. VBT is probably the best place to start, keeping in mind you'll have to get an amp and active xover to driver the subs. Any other sub with an amp built in is really 'mid-fi' at best.