2): I had the same problem but it worked when i pressed Store again after the command. Perhaps there are different versions?

It is a sign that it takes to much current when you turn up the volume. The protection circuit gives a signal to the processor unit to shut down. It could be that one of the pairs IC 400- IC401 or IC 200 - IC 201 are bad. This may show by the large heat sink getting warmer than usual.

There is one more difference when you replace the CPU board on a BM5500 to a BM6500; two resistors on the IR reciver should be replaced. The 6500 CPU board was sold as an upgrade to 5500. On that board it was some extra components to manage the aux port. The most usefull part with the 6500 board upgrade exceept for the possibility to use beolink 7000