I used to play with two guitarists both running AC30 knock-offs WFO. Just love the sound of melting EL84s to bits.
There are so many variables - circuit design, output transformers, voltage, bias etc etc - and valves are only part of it. Things may have changed but back in the day US amps used 6V6/6L6/6550 (and 6416 in the original SVT), whereas British amps used EL84/EL34 and KT88 output valves. The choice was probably down to price and availability. They all use pretty much the same preamp valves, just for some odd reason, Americans insist on calling ECC81/82/83 12A*7.
I don't have a strong preference either way (6550 or KT88) for bass amps. Guitar amps are different - *always* EL84 or EL34, ideally in a combo with two 12" Celestions (combos and separate heads sound different too).