the original Mullards were frighteningly good. (6CA7)

I've not heard much about the new 'copy' but it might be fine. Tubes are microphonically sensitive, by their very nature, just like all electronics, but only moreso.

Only if the original microphonics and the design parameters that were used to deal with those issues, in the new copies..if those things were added in, then the new tubes would sound quite similar to the old Mullards, which were great.

Also, consider the parameters that the amp was specifically designed around. Which exact tube type? those tend to sound best, as the amp might work with various tubes, like car with various tires, but there is only one specific tube, or tire, that works best, with the lowest amount of issues. What problems? Distortion under load.