I'd say they're more modern/american. There are some good clips of the 100 which is just a larger version of the 50 on youtube that can give you a rough idea.
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They're voiced like soldano really and the soldano sound is quite a bit different to both mesa and marshall so i'm not really sure if its either in a brittish or american camp.
Coming from someone who owns the 20, the JCA sounds really british in my opinion.

Could very well be different in the 50 considering that the 50 is designed after the Hot Rod, and has 6L6s as opposed to the 20, which is modeled after the Atomic 16, if I remember correctly.

Very good sounding amp any way you slice it.
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An amp can sound modern and British at the same time. Which is what these amps do.

As long as it doesn't sound compressed(mesa) then that is perfect!
It's pretty dynamic. Of course, if you crank the gain and boost it for br00tz it'll get more compressed, but that's the case with any amp. But in general, it's a nicely dynamic amp.
My JCA20H is pretty modern british.
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