Amp upgrade for Thiel CS 3.7

3.7s are not made for very large rooms, you loose all the bass.People with very large rooms will look at more suitable speakers for this size room.

^Jim Thiel recommended the Thiel CS 3.7’s for an amplifiers up to 600 Watts, and used an amplifier rated at 600 Watts during their development. Why?
 With the appropriate amplification, and especially when coupled with Thiel's own subwoofers; the CS 3.7's can comfortably fill a rather large sized room.
Thiel CS3.7 loudspeaker Specifications | Stereophile.com

By today's standards it is much better to choose a used Diablo 300 vs an old Krell 600, unless you absolutely need separates.Quality vs quantity.

^That is your subjective opinion. Not all would agree. While it's impossible to know for sure, I'll hazard a guess that Jim Thiel would have  stuck with his Krell power amp over the Gryphon integrated. Once again, it depends on the circumstances. One might be better than the other for some, and the other might be better than the other for some others. New is not necessarily better than old. There is also a price discrepancy that can't be ignored. 

There is no way old Krell design could compete with todays Gryphon gear in terms of sound quality.Gryphon was most likely not available in the US when Jim was still alive.At the time it was either Krell or Bryston you didn't have much choice for Thiel speakers.