I would think that setting 'Sub' to beolab 2 and saving the settings ought to work.  What happened when you did that?

We have a Velodyne sub connected to our BV7. In the settings it does appear that there are two subs. It works fine.

And you are using a standard 8pin DIN to RCA lead ? Do you know how it is wired ? 

I am not sure what kind of cable it is. It has a powerlink connector into the BV7. The powerlink connector then has two wires coming out of it each a few inches long with a single plug on the end. I think those are RCAs. Those two plugs then connect to two others that go into another wire about 10 feet long that runs to the Velodyne. At the end of that wire is a single plug, that is somewhat larger. That single plug connects to its mate that then has two short wires coming out of it a few inches long that each have a plug at the end (RCA?) which are both plugged directly into the Velodyne. If you need more detail I can look more closely later, only have time this morning for a quick look.These are the same connectors I used with our Avant for several years. I have not really looked at the cables for many years as the dealer merely disconnected the Avant and reconnected to the BV7 when it was delivered. In the Onscreen menu the first speaker is set to Beolab 2 and the second speaker says the same although it is dimmed out and can not be changed.