I can't speak to Sonus Fabers, but I had asked Thiel about this when I had some old CS3s that I wanted to sand down and stain darker. The thing with Thiels, and many speakers, is that the veneer is so thin that you're taking the chance of going through it to the underlying cabinet if you start working on it--as a result, Tom Thiel suggested I not do it. Not sure how thick the Sonus Faber wood is, but it appears quite substantial. I think you should contact Sonus Faber and ask, they're quite proud of their cabinet work, they can probably give you a good deal of help.
