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

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

Unsound,
On paper you could be right, thus the beauty of a home audition in order to get the actual listening experience. Specifications are helpful but  aren't equal to a hands on contact scenario. We'll soon read his impressions. 
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 

 Charles1dad, As Thiel recommends a minimum of 100 Watts at 2.83 V/m the Aelius would seem to qualify as a suitable candidate, depending of course on desired volume out put and room size.  
  I think it worth noting that instead of doubling power into 2 Ohms from 4 Ohms as a true high current design would, or even increasing power (as spec'd) the afore mentioned Stereophile measurements show the Aelius dropped power, which incidentally would mean that it would be dropping power where the sensitivity of the speaker decreases. To be fair, it might be possible that power from the wall might have been sagging, but I doubt that explains such a degree of disparity.