Having apparently scared you off "serious" DIY (with, I believe, good reasons), I would on the other hand recommend an easy and cost-effective sub-woofer DIY that I believe would be a significant upgrade if you have a Def Tech sub. Parts Express does a kit for a 10" powered subwoofer which they dub the Titanic II (I guess they're not into history) that can be assembled in a couple of hours. Amp and all is a little over $300 and will outperform up to about twice that much. The looks are low-grade and I wish they offered it in MDF for self-finishing but they don't - its basic black and not very nice even at that. If you decide to get it, post for some useful suggestions about assembling it. I'm pleased with mine given that I bought it as a 2-3 year stopgap. Unlike my center channel, I'm even using it with pleasing results with my mains when I'm listening to music. Event hough it sounds like my room is quite a bit larger than yours, I DO have aesthetic considerations too and the 10" gives good performance while being quite unobtrusive.

Sounds like you have a very understanding merchant (although perhaps you have achieved that level of understanding by a high amount of custom). You REALLY should listen to the Linn Ninkas (less than half the price) before making your final decision if not too late.

My own experience has been that "sweet" is a synonym for "lack of accuracy" - although I would NOT say that "bright" is a sysnonym for accuracy. The Thiel speakers, for example, are to my ears considerably sweeter and less accurate than the Linns but in extended listening, it is the "sweet" speakers that I find to be a bit more fatiguing.