The circuitboard provides different settings for different voltages. It's logically be (re)placing some SMD jumpers(s). I would advise to let it be done by someone with electronic skills.

Sorry if I wasn't clear in my original post. I intend on using an external step-up transformer (plugged into US wall socket, EU plug on Beolab 8000's plugged into EU socket on transformer). I'm just unsure what the proper sized transformer would be. I know bigger is better all around in this sort of case, allowing plenty of headroom, but should I be using the max output of 143W as my guide for transformer sizing? Do I only need a transformer powerful enough to run the amplifiers? Or do I need one to handle the total output?

I’m also in the U.S. I’ve been using a 200w step-up transformer with a 220v BL 2 for a few years.  No issues.

I would have thought you will be fine with a step up transformer of 100w if the load of each is 12W will be more than plenty.  I had this the other way round for a while with the some speaker I got from Japan - I used one transformer on each to save running cables, but one one transformer for the pair should be fine.