What is the best solid state amplifier you have ever heard?

Spectral

I don't think it's that hard compare the sound/influence of differing amps, when they are rotated through the same overall system. The caveat being, that all are fully capable of driving/controlling the speakers....and input impedances match with the front end.

Best is Essence except on electrostatic or if a speaker needs some coloring to fix a limitation.  Unfortunately they are out of business and the amps are super hard to repair.

This is difficult because the sound of your system is effected by everything in your system. Now that that is out of the way and given the above, I like the D’Agostino upper end models. I really like the sound of my First Watt SIT-3. So clean and uncolored. 

@on2hifi

Just got to hear a couple classic Threshold amps, I totally agree!  Great sound paired with a nice tube pre and quality source.