I have the FPB 200 amp and I have been very happy with it. The concerns about bass are (I believe) unfounded. I have not noticed any dropoff in bass signal as the lower octaves are reproduced on test equipment.
One of the biggest differences between the older amps and the newer FPB series is that they do not run as hot as the KMA and KSA's. Don't misunderstand though, they still run hotter than my old Aragon.
I have spent a fair amount of time listening older Krell amps and do not think the new FPB amps are of a lesser quality. Some owners of older Krell don't like the 'sustained plateau bias' claiming they did not play in class A long enough.
Brightness is more source or room related than amp and speaker related. Thiel's and Krell should not be a problem unless your source (or more likely your listening room) is overly bright. I have never noticed the FPB 200 to sound particularly bright. In my experience the amp is very well balanced.
The biggest mistake many 'audiophiles' make is attributing room characteristics to their equipment. Is your room sonically bright? If it is your system will be bright. Buying equipment that tends away from being bright will not tame this room characteristic.
Don't worry about this combination, unless the room is a problem. If it is, contact
http://www.rivesaudio.com they can help with the room. Many audiophile would be better served to pay more attention to their room and less to upgrading equipment.