Amp upgrade for Thiel CS 3.7

^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. 

^Just because it might suit you, doesn’t mean it’s necessarily the best choice for all. Some with particularly large rooms, might be better off with more powerful amplification commensurate with the CS 3.7’s power capabilities, and depending on sources might find some of the other capabilities of the Diablo superfluous.

^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

^” No you don’t need more power...”for a larger room?
You might not, but somebody else might.

^There were others, and better than the Bryston’s too. Gryphon had a inconsistent presence in the US for a while, but I'm confident Jim knew what was going on globally. For the reasons mentioned above, I think Jim Thiel would have chosen the big Krell over the Antileon Evo(s) for the development of the Thiel 3.7’s.