Like most of the metal guys here I have more experience with 6l6 amps than El34 amps, although there are a few guys here from the JCM 800 Glory Days I suspect its the same for most of us.
Anyways, what I'm finding is that my current EL34 amp (Rivera K-Tre) is that there seems to be a bit of a "hump" (i.e. sudden change) in volume that you have to get over before things start opening up and sounding good, whereas 6l6 based heads seem to increase in volume in a more linear fashion.
Is this known? Its been a while since I've played another EL34 modern high gainer like a Cobra and I didn't really test those at a variety of volumes.
Also discuss you opinions on EL34s vs. 6l6s in general.
I'm the other way, because I'm over in Europe and have tended towards European amps, EL34s here are more common so I have more experience with them.
The 6L6 amps I have played though I'd say do tend to be along the way you are saying, a bit more linear in the way the react. My Invader on the other hand with EL34s, after you get past a quarter of the master volume range, it doesn't tend to get a huge amount louder (although its absolutely deafening at this level so its by no means a weakness), it seems about 75% of the volume is in the lower 25% of the master volume range.
KT88s seem to be quite linear too, judging by the VHT Sig:X the guy in my last band had, and the Engl Savage 120 my current bandmate has. It definitely sounds cleaner and seems to take a while before any power-amp grind kicks in.
I guess what I've noticed most is that EL34s get a bit grindier earlier (and in doing so get less of a volume increase), and that is a quality I like in my Invader, but on other amps I have found it ruins them due to how the sound is.