AndyMac said:Have a look here there are plenty of Bose amp issues with crackly sound.
http://www.carstereohelp.com/audi.htm
If it only happens at low volume then it'll be a dry solder joint somewhere on the amp. Worth running a soldering iron over the soldered joints where the speaker wires connect to the amp.
I've cured so many of these types of issues just heating up the solder.