I run a pair of the Teacs driving Sonus Faber Concertino Homes. I am using just the center channel Tripath board in each as it is not split into L & R outputs like the other board. This reduces demands on the power supply and results in a very sophisticated sound.
But, I use a tube preamp w/ 20dB gain, the Audio Electronic Supply AE-3 (Cary) and its contribution of bloom is quite nice. I've also run the system w/ a Wright Sound WPL11 also a tube unit, so I've never heard the Teac with solid state or a passive pre.
Because the Teacs are quite transparent, I would definitely recommend a preamp that reflects your taste in sound rather than your budget. You could end up replacing it at a loss, so get the right one first time around.
Just my 2 cents.