I've used 4 different kind of ECC83 tubes in the Sunilda for a while.
1. Mullard 12AX7 long plate, halo getter from 1962
2. Siemens 5751
3. GE JAN-12AX7WA
4 Telefunken ECC83 smoothplate
I find it difficult to describe my perception of the differences, and my preferences could also change depending on mood and music. I also changed the tubes in the 6DJ8 posistion in addition to the tubes in my Eclipse. So no absolute conclusion but I've found a not surpricingly favourite in the Telefunkens. I think the GE's were the lesser ones. The Mullards were engaging, but lacked space around the instruments and scene shrunk compared to the Siemens and Telefunkens. Might be that the combination of Mullard 12AX7 and Brimar ECC88 were just a bit to much The Siemens came with the Sunilda when I bought it, and they are really sweet. I like them! But for me, most of the time, it's the Telefunkens extension, space, inner detail without removing anything of the musicality that make them better than the rest. The combination of TfK ECC83 and Amperex 6DJ8 Bugle Boys is quite special
In the next months I will try to find a favourite 6DJ8 to go with the TfK's