The very highest quality amplifiers are now class D.
https://www.hypex.nl/ , particularly their N-core design.
Comprehensive spec sheet is here
https://www.diyclassd.com/product/nc400/11
And measurements by Siegfried Linkwitz here
http://www.linkwitzlab.com/frontiers_7.htm#G2
But basically 400W into 4 ohms, distortion at 100W 0.0003% (ie 3ppm) frequency independent, and at full power 0.002% (20ppm), output impedance 1 milli-ohm, SNR -125dB. Balanced input.
All from a circuit board in the form of a disc 88mm diameter weighing 180g. An astonishing design.
Oh - and thinking about Sinclair's 1960's class D. It was actually designed by a good friend and mentor (alas now dead) called Gordon Edge. Performance was limited by the lousy transistors that were available at that time, and relied on the loudspeaker to deal with switching transients - which meant that it was an excellent radio and TV jammer.