I noticed the same behaviour with BL6000.
Basically, when you are in LINE IN mode, you NEED to have everything terminated properly, even if there is no RCA plugged in.
For my case, by BL6000 needs to be have Line-in RCA plugged in to the source using a cable, before everything is silent. If you don’t want to plug anything in, you need to short the input via a terminating plug.