More from a tech note by our friends at Mesa Boogie discussing the differences between EL-34 and 6L6 power tubes:
6L6s offer a noticeably different range of distortion threshold and clip potential once you know what to listen for. The rich, full range of the bass frequencies highlighted in 6L6s can generate a wide sonic footprint when pushed, but too much power section clipping with a 6L6 can cause bass “bloom” that expands beyond a desirable sound.