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

It would appear as though the Yisilon Aelius II might not be as suitable below 4 Ohms, where these Thiels spend quite a bit of time.

Thieliste,
You have multiple good choices available as the suggestions you’re receiving confirm. However if you are able to have the Ypsilon pre and power amplifiers pairing , that is one very compelling direction to follow.
Good luck,
Charles

Peter Breuninger has successfuly driven his YG Sonja and MBL 101 MKll with the first version of the Aelius monos.
The Aelius work in class A up to 60 watts wich is probably enough to drive the CS 3.7
The Aelius ll have had a substential revision with bigger tranformers.
Just to tell you guys, here in France many CS 3.7 owners have used the Sugden IA-4 integrated amp with big success and beautiful sound, only 33 watts in class A.

tom_hankins how did you drive your CS 7.2s ?
I might want to add a pair of 7.2s to my Thiel collection at some point but driving them properly is another thing.

Thieliste, 
Michael Fremer reviewed the Aelius for Stereophile and when JA did the customary post review measurements this amplifier was stable at 2 ohms and produced 445 watts into that load.  I don't believe that your Thiels will be a problem at all. I just believe that it will be likely the best sounding amplifier of those on your list. Just my opinion. 
Charles