John,
While others maintain that the J activators are limited in output compared to the other styles due to aperture, etc., you could fool me. While I have no means of direct comparison, I highly doubt you'll lose much going with the Js. I have 'em (see link below) and with the active electronics which are similar to the Europa, sans the tone switches (mine are the EMW package: MV, Blend, Bass, Treble, Filter and 2-P Q switch) it simply kick's *ss! I have absolutely no problem overdriving the hell out of my Eden WT-550 head's tube preamp with Alembic's J electronic package.
As far as trim pots, as far as I know, all activators have 'em for level adjust and or overall balance.
Activator spacing shouldn't be a problem either...it's usually only limited by the size of the body.
http://club.alembic.com/Images/631/8557.html?1093301442 I was looking for a hi-end J-bass as well, albeit a fretted one, and I think I got it and then some.
Good Luck in your quest for the Holy Grail of Tone...you're in the right place, LOL!
Cheers,
Kevin
BTW, Alembic made a couple of basses within the last year or so with full size J-style bodies and J activators that I believe shipped to the UK. I'll be damned if I can find 'em here, but they do exist and there are pix to prove it.