I have a very similar set up: Sony 5400, Thiel 3.6s, Parasound Halo A-21. I recently borrowed a used Audio Research SP-17 line stage from a dealer and was very happy with the improved dynamics and cleaner bottom end (nice phono stage, too). Unfortunately, I couldn't work out a good price on the preamp, but I did find a nice deal on an SP-16, which will be here next week. Like others have suggested, I believe that adding a high quality tubed preamp is going to help a lot with your Thiel/Bryston combo.
A couple of additional thoughts from my own experience. The Thiels have a very strong mid-bass that can easily get out of control if your room isn't ideal. Placing the speakers along the long wall of my room really helped clean up the bottom end.
Also, don't expect much forgiveness from your current set up on badly recorded, over-compressed rock albums...even with a better preamp. Good luck.