NAD makes a pretty good amp, generally speaking. Good at delivering decent amounts of current into difficult loads. I would think the little NAD to be a pretty good match for the Stax SRD-7.
The NAD "house sound" is said to be "a little dark" which I think comes from them voicing their amps to avoid emphasizing the "harshness" and "grain" that some people attribute to class AB solid state designs. I think it a fine choice of "voice" for a reasonably priced amplifier that must make certain electronic concessions in order to meet it's price point. I think NAD tends to choose their compromises wisely and generally ends up with products that are more "musical" than similar products from their competitors. I think it will sound pretty darn good.
Using an SRD-7 with Stax 'phones, I think you will find that you don't really need much power at all from the accompanying amplifier. The Stax set up is quite "efficient" as it were, i.e., lots of dB SPL for few watts. I was surprised at how little power it took to drive various Stax 'phones to loud levels on the SRD-7 that I used to own. Sound quality is quite good as well, though I found it a tad less "refined" than the sound from an SRM-T1. The SRD-7 was not vastly different, really, from the SRM-T1, but the T1 does offer just that little extra bit of transparency and... refinement. Hard to put into words.