Hypex Nilai 500. Low weight product getting great reviews by many but you also see mention of others that weren't feeling it. Alpha Audio has some video reviews with lots of music clips. Comparing the Nilai to other class D but also their Reference Pass Labs amp...all the amps are tested on some high end TAD speakers. I don't have the links but should be easy to find if interested.
These Alpha Audio guys seem pretty legit, they have done some pretty good reviews on Power Supply's, amps, cables, etc. They normally do subjective opinions with objective data (measurements) in their reviews. The Nilai has peaked my interest and I'm going to try one out. It's a new technology so not your standard Ncore/Purifi type modules. It uses a different power supply and FR is linear past 20khz, where as the other models would roll off a DB or so before 20khz. You can try the mono blocks for around $2k us..stereo for $1500. Pretty cheap if they truly perform like many have said...but still a product I would recommend buying with a good return policy in case it doesn't mesh well with your gear. Anyway, just thought I'd mention a small, low weight, cool running amp at a budget type price to go a long with all the other great hi end amps already mentioned. Good luck.