More from a tech note by our friends at Mesa Boogie discussing the differences between EL-34 and 6L6 power tubes:
Sonically speaking, 6L6s are well-known for being versatile and balanced across their entire range of low, mid, and high frequencies. EL-34s tend to offer less bass in low-frequency ranges (in the sub-low regions) compared to 6L6s, but, the higher bass this tube does generate is more focused, is tighter, and is extremely punchy.