Maybe it's the switch to solid state rectification in the power supplies. The old amps that used tube rectification, chokes and less capacitance had more sag, which added more warmth to the sound.
Why don’t tube amps sound like tubes anymore?
When I hear the latest tube amps I’m more reminded of what a solid state amp sounds like than what I remember a tube amp once sounded like. I say that, with most tube amps I hear today, but not all. Gone seems to be the lush tones, warm glow and natural harmonics I used to hear. What I hear is more of a thoroughbred, faster, sharper sound when I listen to a modern tube design today. Then why use tubes?