I have been cycling tons of tubes through my KT88 Firebottle, had a whole thread about some of my adventures. One output tube that I kept coming back to is the Cossor 807.
I have seen them labeled as GEC and Cossor, as 807, ATS25, and VT60. I have also seen them with a brown base instead of the ceramic.
In my system they are consistently the most open, spacious, and enjoyable of all the tubes I have tried. I have 807 tubes from Sylvania, Fivre, Mullard, Mazda, and Firenti. All of them sound good with the right combination off tubes but the Cossor always shine through for me. The accuracy of timbres and revealing quality is addictive. I use a 5u4gt rectifier either an RCA or Electro Harmonix, I can't tell any difference between them.
The input tube has a dramatic impact of course. My go to inout tube is the 14n7 clear glass. It has a very similar character to the Cossor tubes. The combo doesn't have any weaknesses tonally. Highs, mids, lows, are all where I want them. I believe the 14n7 isn't just the 12v version of their similar looking 7n7 or regular 6sn7. It sounds deeper and clearer than the other Sylvanias. I'm pretty sure it is the 12v version of the sought after metal base 6sn7w. I don't have one to compare sonically but they look the same to me, down to the copper support rod.
If I want to warm things up I can use the grey glass 14n7 or the MOV CV1102.
I never thought I'd say this but I think I'm done tube rolling with this amp! It sounds so so good! Ironically, I actually have another amp coming next month. This one isn't made by Dennis, had Thomas at Ultrasonic Studios make an amp to my demented specs. It is a push pull amp designed specifically around tube rolling. I'm going to keep my KT88 Firebottle as my single ended amp. have a feeling that the push pull simply won't sound the same. I have talked to Dennis about the possibility of another amp in the future but we will see...
Anyway, I'll keep checking in here to talk about these wonderful amps but don't be surprised if I become scarce next month when my new toy comes in lol.