Usher S-520 amp pairing

I bought a pair of the 520's for my daughter and tried them in my system for awhile before shipping on to her. Compared to any of the other speakers I own the Ushers are a little bright so for my ears they need some warmth upstream.

I don't have any experience with the specific NAD or Cambridge amps you're considering but I'm a fan of the Decco. I use one as a preamp/DAC with an external tube power amp in the winter and as a stand-alone integrated in the summer. The versatility and sonic quality are great. The build quality is about the same as the Cambridge integrated I used to own, or the NAD stuff.

A lot of people love NAD gear but I had reliability issues with three out of the four pieces of NAD gear I've owned.

Sfar is correct in that the 520's need some warmth upstream. With solid state gear, I would recommend heavy copper cabling throughout. If a subwoofer is going to be used, it would greatly balance out the range so the upper end will not come across so hard.
Regardless, the Usher 520 is one of the best mini-monitors I have ever heard. The midrange will make you melt in your listening chair.