The 7868 is identical electrically with the 7591A. They're in between a 6V6 and a 6L6GC for power output, and they have a lot of gain for a power tube...almost as much as n EL84, so the preamp can be low gain and still drive them well. They're my favorite power tube personally. EH makes a new 7868 that sounds good, though NOS of course sounds better. The transformer set will work fine with the 7868, 7591A, or even the 6L6GC for a slight mismatch. The 7591A's were used in a lot of Ampeg stuff so you can look to those for circuit ideas, though they ran them conservatively and you can get a lot more out of them if you run them like Fender ran the 6L6GC.