There probably has been no fundamental change in tube technology in 60 yrs. There have been some improvements in convenience (e.g., remote control), but, the circuitry remains the same, and as for parts quality, there are plenty of older parts that beat anything available today.
When it comes to modern vs. older tube designs, it is more a matter of voicing than technology. Many current tube linestages are being specifically designed and voiced to be more like current solid state units. It is very much a matter of taste and system matching as to whether this is good or bad.
If you are looking at something 10-15 years old as the used alternative, I would say that you would not be giving up anything at all, in terms of sound, and would be way ahead when it comes to a dollar for dollar comparison. The only issue would be whether some of the components in the older unit have deteriorated. Specifically, one would be looking at the electrolytic capacitors in the power supply. At 10-15 years, these will probably still have quite a bit of life remaining. If they do go bad after some time, any competent repair facility can replace those parts. That is one of the positive attributes of tube gear--all of it can be serviced and kept up to original quality (it is often harder to service solid state because replacement transistors can be really hard to come by).
When it comes to modern vs. older tube designs, it is more a matter of voicing than technology. Many current tube linestages are being specifically designed and voiced to be more like current solid state units. It is very much a matter of taste and system matching as to whether this is good or bad.
If you are looking at something 10-15 years old as the used alternative, I would say that you would not be giving up anything at all, in terms of sound, and would be way ahead when it comes to a dollar for dollar comparison. The only issue would be whether some of the components in the older unit have deteriorated. Specifically, one would be looking at the electrolytic capacitors in the power supply. At 10-15 years, these will probably still have quite a bit of life remaining. If they do go bad after some time, any competent repair facility can replace those parts. That is one of the positive attributes of tube gear--all of it can be serviced and kept up to original quality (it is often harder to service solid state because replacement transistors can be really hard to come by).