like i said, the sound of an amp has less to do with the tubes than with topology.
electrically speaking, the 6V6 and EL84 are fairly similar except for a couple of things - the EL84 is driven easier than a 6V6. so - ALL OTHER CONDITIONS BEING THE SAME - the 6V6 amp will be a *bit* cleaner than the EL84 amp - or rather, break up later. i can vouch for that because i built an amp which would take any output tube you gave it and had a switch to choose between 5 cathode bias resistors. everything else being the same, the 6L6 had the best cleans, the EL84 had great chiminess and the EL34 was just seriously ballsy. i felt the 6V6 was the best allrounder with better cleans than the EL84.
however, this is barely a criterium, because we haven't even touched upon that horrible B word - biasing. then there's screen grid biasing to consider. in practice, all other conditions are practically never the same, and the comparison is often moot.
the EL84s do play a part in the "AC30 sound", but a pretty small one compared to three things about that amp: a. the abscence of negative feedback b. the notch-filtering effect (unintentionally) introduced by the tremolo channel and c. (HUGE role) the alnico speakers these amps used to be fitted with as stock speakers. bit of a shock, eh?
anyway, long story short: i would not choose (or consider) an amp based on the tubes in the power stage or generalize things to this extent... the simple reason for the EL*4 being associated with "the english sound" and the 6L6 with the american sound is simply because that the EL84/34 was an english tube and the 6*6 american, so people used what was available easily at the time. since then, things have changed massively.
so let your ears be the judge; this is guitar amplification, not surgically precise tube hi-fi!