Hi, a BL2 is normally connected via powerlink cable. The powerlink cable has a volume regulated line signal and a tricker to turn on the build in amplifier.
The mini jack socket for head phones is also a volume regulated signal - but I do not know if it is the exact same volume level as you find in the powerlink signal.
However, the tricker signal is missing. There are special cables on the market (e.g RCA > DIN) where the tricker signal is separated in e.g a USB cable in order to turn on B&O speakers when used with a computer or none B&O television. I have not tried them myself. The company AV connection is showing demos on Youtube.
But I don't see what should deliver that tricker signal when BV5 is turned on/off and I dont know if the BL2 could get damaged or sound distorted when using signal from head phone jack rather than powerlink socket.