Jdec - The hexfreds have a completely different job. The rectification stage in an amp's power supply helps to regulate the voltage going into the amp. The Hexfreds are upgraded diodes (which every amp has). These diodes essentially keep the signal flowing in the right direction (sorry I'm not an engineer but that's the jist of it). The cool thing about the hexfreds is they do their job faster with less recovery time, which translates into increased clarity during complex musical passages.
Transl - The SLI-80 can use a wide variety of power tubes including EL34s and 6L6GCs. I don't have personal experience rolling those tubes (I just don't have as much time to play as I used to!) but we have a few customers using EL34s with great results. An EL34 won't have quite the same kind of bass authority you'd get from a 6550 or KT88, but the midrange will be prettier. There's always a little trade-off when switching power tube types, and you just have to determine if the loss is justified by what you gain.
Transl - The SLI-80 can use a wide variety of power tubes including EL34s and 6L6GCs. I don't have personal experience rolling those tubes (I just don't have as much time to play as I used to!) but we have a few customers using EL34s with great results. An EL34 won't have quite the same kind of bass authority you'd get from a 6550 or KT88, but the midrange will be prettier. There's always a little trade-off when switching power tube types, and you just have to determine if the loss is justified by what you gain.