interesting.....mine doesn’t do that...must be the 34s...Even now, a couple of times, while working on my amp circuit again (now 30+ years later, and now a bass circuit), and while trying to get the cleanest low E from a 2x6L6 amp, I have said to myself "Wow, that is the best low E yet!", before remembering to correct for this effect.
[P.S. Did get a clean 40Hz by wiring the output tubes as triodes (usually a hi-fi idea), but it only makes 20 watts.]