Should I buy used Thiel CS5

Start your search for a krell FPB 600...

Will a FPB 400 CX work?

rather demanding amplification

I think that's a bit of an exaggeration made mostly by Stereophile.  I currently have a 150W amp and it is driving the Thiels fine.  They can go pretty low.  

I think that's a bit of an exaggeration made mostly by Stereophile.

A 87db 150w is fine (even 100w) , what Stereophile means is you need good current to back up that 150w  so it can do up to 300w into 4ohm and try to get near to 600w into 2ohms. (If you want the best from them)

" it can be seen that the CS5 presents an amplifier with a pretty severe load. To say the least—"current capability" if the sound is not to become anemic."
https://www.stereophile.com/images/archivesart/690CS5FIG01.jpg


Cheers George

@jeffvegas, Yes, a Krell FPB 400CX will work. Should work very well.