Mine also always dim, since forever. Reason unknown. Either the manual is false, or the firmware has been changed at some point and is making the soundtouch not acting as it should. Also this issue has been brought up multiple times over all the the other non-english forums.
There are so many quirks with software for the bose products that this alone would be a reason to stay away from bose in the future.
I'm sorry to hear you're also encountering an issue with the LED on your system. Do you also have the SoundTouch 300 Soundbar, and has it been working properly with your TV to play audio, even with the LED dim? Also, are you experiencing any other particular problems with your system that we could help with?