I run 4 SED =C= El34s with 2 SED =C= 6L6s in my Herbert and it sounds fantastic. I had only used 6L6s in my amps before (mostly Mesa), but hearing the EL34s has converted me to start switching the 6L6s out in everything. I still like the bottom end of the 6L6s, which is why I'm keeping a pair in the Herbert, but the EL34s have such great punch and tone, they sound a lot more "alive" to me than the 6L6s. Running the one pair of 6L6s allows the EL34s to dominate the sound, but still get a little more low end grunt. If I had to only run one type of tube in the Herbert, it would be the EL34s, but seeing as though you can bias each pair seperately, it gives you the option of mixing it up a little.