After reading this site for years and years, and remembering back to the days when slide rules were the bomb in calculating things for amplifiers… I'd say that most of the differences between the eras can be boil'd down to:
• economics of various parts
• esthetics of design choices
• fads and magical elixers
• sentimentality and purism
Economics is one of the 'real deals': inductors used to be cheap, now they're expensive. Capacitors used to be really expensive, now they're cheap. Commodity tubes used to be dirt-cheap, now they can be scores of Euros. Even solid-as-tanks resistors once were pretty expensive, then they got cheap. Now they're even cheaper, but no longer 'tanks'.
esthetics clearly is an evolving thing, but it is also does guide generational differences. Today, we have open-frame blue underlit tubes; people use gas regulators not for their practicality, but their telltale glow. Bespoke volume controls are au currant, but once it was silver-plated speaker banana jacks, and before that bakelite knurled knobs.
fads & magic has always been part of what defines the generational differences. Today the Aikido design (which is admittedly both novel and esthetically cool) is on the wane. Its still very good, but times change. Once it was SRPP (which no one can remember what the acronym means/meant). Before that there were cascodes and fairly unique common-grid designs lifted from the HAM folks. Times change. E'n the decision as to whether input or middle stages benefit from pentodes versus triodes (or FETs!) is contentious… and fluid.
sentimentality remains perhaps one of the most influential factors though. If we are honest, building tube equipment is a man's hobby, mostly. (Sorry, Katje! - you're extraordinarily special!) Just like rebuilding turn-of-the-century automobiles and trucks. (19th to 20th, not the recent one!). Design choices are very often rooted in original spec sentiments, then gradually 'honed' with decidedly later innovations. Sometimes openly, sometimes hidden below the hood.
Intellectual avocation (not in the bullet list) is also another significant factor. Many of the people who congregate here have both remarkable intellectual talent and the desire to dig deep, deep into the analytic underpinnings of abstract electronic engineering and physics theory. For many, just to understand, to 'grok', and to then design from the ground up - NOT depending on copying others' designs - is hugely satisfying. Evidence that all that studying, noodling, calculating, driving the wife nuts, is worth it.
Just like (mostly) gentlemen who are maybe bankers or attorneys, or civil construction generals, but who in private life have cultivated a life-long hobby of fine cabinetry and woodworking, or musicianship, or photography and printmaking, or crafting something really oblique like fountain pens, or cuckoo clocks, or distilling whiskey on a tiny scale. It consumes the wan hours of a lifetime, and the results, the creations are tangible. Evidence of intellect. Good stuff.
And guess what … all those things are also influenced by the prior 4 bullets.
So that's what influences the generation-to-generation designs of … everything.
GoatGuy