Metralla, I also have a pair of the TJ (Full Music) meshies (i.e., the ones with the globe shaped bottle as opposed to the princess or "ST" shaped bottle). In comparison with the Sophia Carbon Plate and Western Electric 300Bs, the TJ globe meshies sound a bit thin in my system (that is, lacking in weight in the upper bass and lower midrange frequencies.). Both the Western Electric and Sophia Carbon Plate 300Bs are notably better sounding output tubes in my amps.
As for the comparison between the two solid-plate tubes, the Western Electric 300Bs have that magic midrange, but they lose on just about every other account to the Sophia Carbon Plates. The Carbon Plate tubes are significantly more dynamic and much more extended at the frequency extremes than the Western Electric 300Bs. And the Sophia Carbon Plates give up very little ground to the Western Electrics in the midrange department. As a result, my Western Electric 300Bs sit on the shelf collecting dust while the Sophia Carbon Plates get all the air time.
As for the operating requirements of the Sophia Princess 300Bs, the Carbon Plates are a solid-plate design that is 100% interchangeable with standard Western Electric spec. 300Bs.
However, I'm not certain about the Sophia Princess Mesh Plate 300Bs. The Sophia Electric web site seems to indicate they are 100% interchangeable with normal solid-plate 300Bs. However, I recall that the TJ meshies required a reduction in the plate voltage. I think it would be prudent to call Richard at Sophia Electric before running the Princess Mesh Plate 300Bs in an amp designed for a standard Western electric spec. 300B.
How does the above description line up with your listening experience with the Full Music meshies and the Western Electric current production 300Bs in your audio system?