I had mine installed in my 350z and it was a bit of work to get everything finely tuned. The + and - are for volume control in SD. I don't think there is any way to control brightness though (or at least I have not found them) accept for using the gesture V for night use. One problem I did have is that the Vinyl software kept setting my speakers to quadraphonic which made the sound pretty awful. I ended up removing the software and just pointing windows to the driver files and then let windows manage the sound hardware. I did not have the same problems with the MP3/CD sound, but when I get off of work, I will look at my settings and post them. The sound problem you mention (for amplified) sounds more like a hardware problem than something from SD.
Maybe soon we can get the manual and start really using the INFILL hardware like the small touchscreen.