I have been using a synchronisation cable between my BL5s, but couldn't get it to work properly. In the end I started wondering whether something was wrong with the connection -- but I started by getting a new cable from my dealer.
As I'm using various non-B&O sources, a synchronisation cable is recommended. I didn't have cause to be dissatisfied with my bass previously, but now it's become extremely precise, and richer-toned than it used to be. This has also had consequences for the overall soundstage, with a more precise placement of sources (as if that was possible.)
In other words: make sure you're using a synchronisation cable if you have non-B&O sources in your setup.
Parts Nr 6270901
(This tip is for when you are using non-B&O sources, or a combination of B&O and non-B&O).