Problem solved –> user error due to inexperience,
For benefit of others:
- Unterminated line-in WILL give hum / buzz –> this prevents speaker from going on autostandby.
- Poor quality line-in (eg. mini jack to RCA) connected to crappy source (eg. laptop) WILL give hum / buzz as well.
- Connecting a decent rca plug to a decent pre-amp got things working well.