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

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.

Thiel is terrific,
You have assembled a very worthy list of amplifiers.  I've heard the D' Agostino and the Ypsilon amplifiers on 4 occasions between the RMAF and CES. Show conditions no doubt but in good rooms that were very well setup on each occasion. Sometimes you can get a feel for the brand's "house sound" generally speaking. 

The Ypsilon is the more impressive in my opinion.  It comes across as more natural, musically involving and ultimately more interesting and captivating compared with what I heard from the D' Agostino demonstrations.  Ypsilon products consistently sound exceptionally well when ever I've heard them. 

You're planing to audition these various amplifiers which is near mandatory at this elevated level.  Obviously hearing them driving your Thiele is the ideal scenario.  I look forward to your listening impressions. 
Charles