From my experience, I'd say that in power amps, newer and higher power transistors has enabled more power from less transistors. The distortion results and tendancy to crossover distortion has reduced dramatically as a result, along with the fussy lengthy warm-up process as the transistors struggle to thermally track each other.
As for preamps, quite competent circuits can be made on high grade op-amps now and the best valve ones don't need to be complex either (Glenn Croft anyone?). Older stuff was often limited by the quality of components.
Tear down these walls; Cut the ties that held me
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