Teddy_fr:
Hello Simsim and welcome back to Beoworld!
Powerlink signal is not balanced, only a line output. (For more details you could have a look at Beotech).
I use powerlink to XLR cable in the opposite way as you ie from powerlink sokets of a Beocenter 9500 to XLR socket of an amplified B&O Studio monitor, but this is exactly the same thing.
Here is the diagram of my Xlr to Powerlink cables. Their length is approx 15 meters and the sound is perfect, no hum, no distortion!
I'm not sure about the shorting of pins 1 & 3, it'll be fine when wiring an unbalanced signal into a balanced input, but in this case you are suggesting shorting a -ve output to ground - this may be OK if the output is transformer coupled but that may not be the case and you could feasibly damage your device if you are unaware of it's output circuit configuration!
If you are going to make a modified lead I would suggest using pins 1&2 of the XLR as your two wire connection and not short anything to ground.
See HERE
Even easier, use the subs RCA outputs!
Generally speaking, you aren't learning much if your lips are moving.