Dan D'agostino classic stereo amp + Dartzeel NHB-18NS

tube like warmth

??? That means colouration to me. It should sound the most neutral, compared to others.
If it behaves anything like an LDR based passive volume control which it sort of is, it does have a small gain section which I believe can be switched out, then it should sound like a direct source to poweramp connection, a transparent and dynamic sound, with the least amount of colouration, given a great impedance match, that you have.

Cheers George

D’agostino Classic Stereo power amp

That amp would be great to drive the OP's Thiel 3.7 evil load with, should sound great.
http://www.stereophile.com/images/archivesart/1208T37fig1.jpg

Cheers George

NHB-18NS’s while the output impedance was a high 2500 ohms at low and midrange frequencies, rising to 3080 ohms at 20kHz, the output impedance a little high for some power amplifiers.

The NHB is sort of like a highly glitzed up Lightspeed Attenuator, it’s output needs to see to see an input impedance of the poweramp to be at least 33kohm or higher. The input impedance of the D’agostino Classic Stereo power amp is 100kohm, so you have a great match. Should sound very good.

BTW: looks like a powerhouse amp. http://www.roksantrading.com.hk/upload/201601/06/201601061314126789.jpg

Cheers George