Appearances can be deceptive, they are the only brand I consider for amplifiers. Personal preference of course, but for me the NAD sound is the perfect balance of refined detail combined with effortless slam and impact.
They offer all of the features you could ever want in an amp, pre outs, sub outs, well designed and bypassable tone controls, very healthy power outputs which are much more impressive than the on paper figures, plenty of inputs, remote control, nice sounding headphone outputs... For me they give you everything you could want as well as clean powerful refined sound.
Second hand I'd check out the C370 amp. It's one of the standout NAD amplifiers for me. Just try and get a serviced one for piece of mind as they are getting on a bit.
New ones I can vouch for the C326BEE, that's what I have now and I love it. In comparison to my old C350 the newer amps sound a bit more forward in the upper midrange and a bit tighter in the bass. Generally a more open and possibly more accurate presentation. Still typical NAD in the bass department (lots of it).
Also many now come with built in DACs and various digital inputs which in today's world is a very useful feature and it'll be the way everything goes eventually.