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

Well the customary method is you disconnect it and swap it out with another amplifier. Just be sure to swap only the amp. You must use the same power cord, and cables, and cones, and even the same fuse if an aftermarket fuse is involved. Otherwise if you change the amp and the power cord, or cables, or anything else, then you will never know for sure how much of what you're hearing is the amp or something else. Changing only the one component allows you to hear differences due to that one component and that one component only.

Well put, @millercarbon.

Among the very best Solid State amps I’ve heard is my own MOSFET-based Belles SA-30, the MC² S1400, and the now discontinued Crown Studio Reference II.

In the opposite end of the power scale (compared to the latter two amps mentioned above) I have heard very good things about SIT-based amps, and though I’ve yet to listen to any iteration a few people I know well (and who’s ears I trust like my own) hold them in high regard.

I had a Bedini 25/25 about 25 years ago and I agree it was the best. A friend of mine was doing renovations for a lady in Manhattan and she threw it out with her garbage. My friend saw it and asked if she was really throwing it out and asked her if he could have it. He even offered her money for it, but she told him to take it and did not want anything for it. He used it for a short time and then gave it to me for free because he went to a Marantz tube amp. I also went to a tube amp later on but I'll never forget playing the Los Lobos Kiko ( CD ) threw that amp after a half hour of warm up. The sound and soundstage was amazing. The only reason I switched amps was because I couldn't play it loud enough because my speakers needed a little more power. I should have kept it though. I still regret getting rid of it.

After ~ 15 years, I still think fondly of the 47 Lab Gain Card amp wth 2 Humptys

My Audio Alchemy DPA-1 (175 wpc class D) is a total bargain

The FM Acoustics 1811
At Munich High End this year. To me, it is bar none the best transistor amplification on the market. Quite possibly the best amplification period. That room was best at show. After it, everything else was boring.

Bakoon Amp 13R.  A remarkable amp at 25 watts per side. This is where my amp search ends.