Hi Fredman ... different thread, but I can also respond to this one. I own spica angelus and I had the same problem ... wonderful midrange and imaging (thrashed Spendor s3/5 in imaging) but not enough "drive" to get my toes tapping.
I added a REL Strata3 and now the system sounds absolutely great. The downside was that, even used, it cost $800. Add the $800 to the $250 you'd get for the tc-50s and you'd have over $1000 to throw at new (or used) main speakers. It's a real trade-off, since $1000 buys some good speakers. I have seen kef reference 103.4 and Epos es-25s for less than $1000 used, both of which would probably beat the tc-50 + sub combination, IMHO.
So in summary a REL strata 3 will definitely give you the bass and drive you want, but unfortunately at quite a high cost relative to the rest of your system. On the other hand reselling the REL is extremely easy, so it won't really cost you $800 if you sell it in the future.