PS Audio BHK 300 mono VS Modwright KWA 150SE mono VS D'agostino Classic stereo

The good thing about the D'agostino Classic is that it doubles down all the way to 2 ohms, it's not the case of the other brands.

The D'Agostino shouldn't be troubled by any of those 3.
Is Gryphon an option? Or, perhaps D'agostino era Krells?

Well i did audition the Gryphon Antileon Evo but at 35K€ for a stereo amp that's very steep!
I'd prefer staying within the 25k€ range for monos or 16k€ for stereo amp.
I don't want to go the Krell route as i don't find them refined for Thiel speakers.
The Aesthetix Atlas (already auditioned) are in my short list until i demo the Ypsilon Aelius ll in 2 weeks.

Gryphon hasn't had a US presence in some time, I knew they were pricey, but just not how pricey. I can certainly understand your position. I've always like those Krells with Thiels. But, hey, it's your money. 

I'm familiar with D'Agostino Momentum stereo and it should drive  Thiel 3.7 based on specifications.

You should add Hegel H30 to your list.  I have one and is superb.  Again, base on spec it should the Thiel 3.7.   There are many professional reviews available online that describes the SQ accurately.