I'm considering buying an M1+1 upgrade kit for my M1. Does anyone have this upgrade, or know about it? What additional multisamples it provides, how hard to install, etc? Thanks very much.

I would advize against it. Unfortunately it is not made anymore, and they have frequent problems in my experience. The sounds are great though. About six months after I got the first one it started making odd noises, static, odd tweaky tonal things, basically making it unusable. I didn't happen all the time and was worse when it was cold. The shop where I had it put in, put in another one for free, and alas 6 months later, same problem. Now I can do nothing about it cause they don't make em anymore so I have an expansion I can not rely on because it sometimes workes and it sometimes doesn't.