Personally I would stay using the MCL (Master Control Link)
I am guessing that the "Bang" is caused by the fact that there are incorrect connections in the BeoMaster 7000
The first thing I would ask is the cable from Beomaster 7000 to the LCS9000 a B&O cable or made either by your friend or purchased from someone other than B&O ?
Does the cable look like the small picture below ?
Is the "Left" and "Right" plug the correct way round in the BeoMaster 7000 ?