I also forgot to state above that each tube can work best with a different transformer primary. For example, in an ultralinear push pull circuit, both the EL34 and the 6L6 work very nicely with a 6.6K primary. In other circuits or with other tubes the optimal primary will differ. So it is really impossible to say what tubes will work well unless you know the bias point, the transformer primary, and the circuit. Just because you can plug a certain tube in and it won't blow up doesn't mean it will sound good. Also, you may prefer the KT77 in one circuit and the KT66 or 6L6 more in another. So if you read that someone likes to bias their EL34 and 40 mA, it means absolutely nothing if you don't know the plate voltage or the circuit, or whether it is fixed or cathode bias or the transformer primary, etc... In general, I don't really like the EL34, but maybe there is a circuit where I would like it more than anything else.....