More or less, every current production tube amplifier is a knockoff of one of the three classic designs.
Sonically, I find some of the differences in new and old are related to:
1) A lot of the older designs used a lot more circuitry
2) As people had less money back then, many older amps are biased colder to prolong tube life
3) Older designs used carbon resistors and paper in oil coupling capacitors to produce their richer sound. Modern amps generally employ metal film resistors and polypropylene coupling caps, and sound more forward and insightful
Sonically, I find some of the differences in new and old are related to:
1) A lot of the older designs used a lot more circuitry
2) As people had less money back then, many older amps are biased colder to prolong tube life
3) Older designs used carbon resistors and paper in oil coupling capacitors to produce their richer sound. Modern amps generally employ metal film resistors and polypropylene coupling caps, and sound more forward and insightful