Are you running a sub? It is hard to believe the NAD does not have the ooomph. Crossing to a sub, and even better if you roll off the amp input bottom end can really open up some speakers.
Maybe because the amp is it is very clean and those SF's are , well drool drool, smooth and not over emphasized. Have you played with any eq? It could be you are accustomed to a little midrange boost so it sounds flat?
You are going to have to audition in your room as at your level, it is not a difference of quality, but of preference. It could also be you need to let your brain re-program" to the different eq of the Sonas Fabers and then you'll love the NAD again.
Always think eq first before you blame the electronics.