Update - success at last!
Just letting y'all know that I finally got the armboard (and Achromat) last Friday and was able to put it all together over the weekend. The arm-hole in the board was indeed slightly off (about 0.5cm) in the old armboard, so it must've been drilled for a different arm after all.
Anyway, it sounds fantastic now! No channel balance issues (that I've noticed so far anyway), no tracking issues (I've yet to determine just how well it tracks, as I've only listened to a few records so far, but it's sounding good), and really gorgeous, solid, stable sound with incredible inner detail. It now sounds vastly better than my [very good] CD front end.

I suspect the Achromat has a lot to do with the improvement.
I'll keep you updated with any further developments, but I'm hoping that'll be the end of my turntable strife... I still want to get the Vector/Anniversary upgrade eventually, but I'm not in any hurry - I'm perfectly happy to put my feet up and enjoy some music for the time being.
I am a bit put off of Funk Firm from my experience - it's taken nearly three months to get a handful of spare parts (user-assembled, no less!) and a stock item (the Achromat). The only work needed (AFAIK) was drilling and finishing the arm hole. But I'm not completely put off - Arthur K is a very good guy to deal with, and from the sound of it he has had a rather unfortunate winter! I'm willing to give him another chance with the Vector/Anni upgrade - I'm just glad I won't be doing it for a while!
Dunc