It all seems to depend on how you define good guitar sound and/or what kind of music you are trying to play. It also seems to be generational; not in terms of the players, but in terms of the music they're playing. Guys who play nothing but modern death/black/thrash/grind/whatever-core metal tend to prefer the 6L6's because of the increased total bass response and the more severe highs. Guys who play rock/metal from before Y2K or who play any leads tend to prefer the EL34's because of the better mid-range harmonics, tighter bottom end, and more articulate crunch. Exceptions are plentiful, but that seems to be the trend.
Regardless of tweaking capabilities, there are signature classic tones to both a Marshall stack and a Mesa Rectifier that people know and love, and these generally represent what people think of when they mention EL34's or 6L6's in a rock or metal context. Perhaps in future generations of amps we might see improvements in the hybridization or at least in the modular employment of both types. We might even dare to dream that, at some point, some reliable company might actually begin to produce significant quantities of high-quality tubes of all sizes again. As it is, the mass-produced ones generally don't hold a candle to the best NOS tubes from decades ago, and the really good quality tubes (NOS and the equivalent) are sold for ridiculously inflated prices considering the very simple and completely obsolete technology they represent because they are no longer mass-produced.
