. . . 20 years ago, semiconductors could not handle as much power as the ones made today; more were needed for a given power output. Today these devices can handle more power so, at least on paper, less devices for the same power rating should yield better performance. I’ll be willing to bet that if the Belles 250 had today’s semis, it would sound even better IMO.
I agree. The overall circuit designs don’t change very much, but the quality of the component used have much improved over the years. Electrolytic capacitors not only age over the years to the point where they can make a once great sounding amp, preamp, etc,. sound down-right bad now. New generation high-quality electrolytic capacitors are much better in all respects and last longer than the best that were available 20 or more years from the past. Also, many older amps used carbon-composition emitter resistors which rapidly age and will change the overall sound of the power amp. Now, amp builders often use non-inductive wire-wound emitter resistor which don’t degrade over time in their highest dollar SS amps and use quality metal-film emitter resistors in their mid- to lower-price amps. Finally, you can now buy a group of quality output devices (transistors) that are practically matched out from the box. Years back, many hi-end amp builders would buy a large quantity of of output devices and curve-match the for each amp - a very time consuming process. Early generation Threshold power amps comes to mind as that’s what they did. Now an amp builder can just place an order for a few thousand OnSemi (formally Motorola) devices and install them right out of the box with confidence.