erda1:All you need to do is connect the sub out from the 950 to the velodyne's left channel rca input, using a single audio cable. If you have an older Velodyne sub without a variable crossover, the internal crossover is set to 80 hz (unless somebody had it specially modified). Set the crossover to the same (80hz) on the 950. Do the set up with the 950, setting all speakers to small, check and set speaker levels; you may need to also adjust the velodyne's output level knob. I have the Velodyne ULD-18, and it's set up this way. Even though this set up is using two crossovers cascaded, I don't really hear any problems.
Homieopatrick: You don't need a "y" cable. All subwoofers are mono. Thus the 950's subwoofer out into either the left or right rca input on the sub will suffice. The earlier Velodynes marked their "left" input for use as a mono input.
Curegeorg: run speaker wire from the preamp?????? There are NO preamps (especially not the 950)that you can run speaker wire from!! There are only rca jacks. Methinks you may be thinking about a receiver without a sub output.