Leben CS600 cannot drive a Stirling. I know it because a close buddy has the Kensington and he was using CS600. It simply doesn't have the grunt. Consider that Kensington is easier to drive than Stirling because of its alnico magnet.
I personally use Turnberry SE. Pass XA-25 will happily drive your Stirling but you will miss the touch and lushness of Leben. I would suggest a Rogue Stereo 100 or Audio Research Ref 75se. A proper 70+ watt tube amp which can be used in triode mode (not ultralinear) will give you the sweetness and power you are looking for.
Here is an interesting review for you