From my Road Kingin' experience with Mesa tubes.
2x6L6 vs 2xEL34.
6L6 - Nice big bottom end. Smooth highs. Less pronounced mids. Fuller, even sounding tube. Hold together better when driven hard (more headroom). Better overall clarity.
EL34 - Less bottom end. More highs. Well pronounced mids. Thinner sounding. When driven hard they can get squishy in sound and feel (less headroom). They sound mean with gain :evil:
Tube characteristics can change with different brands.
