I have had the same problem with my Thiel 2.3s and yes a tube preamp does go a long way to control the bright top end of the Thiels. I am currently using a Krell KST 100, VTL 2.5 Preamp with MIT MH750 cables with excellent results. So before you replace your Amp try using MIT MH 750 speaker cables. Will the cable change be enough? Maybe, but give it a try. You can get them at Audio Advisor at 50% off and it they don't work return them within 30 days for a refund, no questions asked. Just be sure to break in the cables before making a final judgement. One final thought. If you do replace the Amp be careful what you use and try if possible to audition it in your system. Thiel 2.3 are a real pain to match correctly. I tried using a BAT VK 200 and BAT 3i and thought this combo was the ticket, the sound was way to bright. A terrible match. Also tried the Aragon 8008st, much better but my current combo seems to work best. The Thiels do sound very good when matched correctly, its just very hard and at times fustrating to get it right.