One of the most important things to consider about an amplifier is the impedance and sensitivity demands of your speakers. If you are running a pair of Wilsons or larger Thiels, for example, you need a high powered amplifier that doubles its output with each halving of the impedance. This would cut down your options by 90% or more. Very few amps do this and the ones that do are all heavy, expensive solid state monsters. Modern examples are from Pass, Krell, and Gryphon.
On the other hand if you run (or expect to run) a pair of high sensitivity horn speakers then a lower wattage tube amp, perhaps with SET design, would be a logical choice. An 8 watt tube amp is not better or worse than a 400 watt solid state amp - they are made to drive different types of speakers.
Regarding technology and design the big thing now is Class D. These amps have improved dramatically and some models are now the lowest noise amps ever made.
I realize that I'm going to get whacked for saying this but a good place to get information about the root of your question is Audio Science Review (let the deluge of hate begin). Amir does some of the most thorough testing of audio components of anyone out there and he distinguishes well engineered gear from gear that isn't as well designed. We can argue all day about whether ASR's measurements correlate to sound quality but I am personally interested in how a product measures and how well it performs its intended function.
Speaking from my perspective as an audiophile geezer, I don't see very much that is truly novel in the audio world. 90% of the progress is the refinement if old technologies using better parts, exotic materials, smart electronics, and improved manufacturing techniques. Once in a while something comes along like Class D but mostly manufacturers are tweaking old designs and making up new marketing stories to differentiate their products.