You might want to consider the Vandersteen VCC-1 center channel speaker. It has a single, high quality 6.5" coaxial speaker, so it provides a "point source" type of sound that should match well with the Thiels. The VCC-1 also has a proximity switch (on the back) which allows you to contour the sound for how the speaker is placed -- close to a boundary (like on top of the TV), or stand-mounted in front of the TV. With regard to the side/rear surrounds: you didn't indicate if you want dipole or direct-radiating models. I am currently using a pair of Coincident Technology Tiumph Signature speakers as rear surrounds, and am very pleased with them: above-average efficiency, very clean, and close to full-range.
