I don't call it an improvement one day maybe when technology has let them get to use switching frequencies 10 x higher than now, but class D is the only real new technology I can recall.
Everything tube or transistor although bigger, heavier, and better built these days, can date it's base circuitry back to the 1940's 60's and 70's things like Williamson tube designs, and same goes for SS.
Cheers George
Georgelofi
Even class D technology dates back to the late 50s. Although as far as real sonic improvements, those may be dated after 94.