Marco, congratulations on the CSLS!
I don't know if it inverts polarity in the signal path between its RCA inputs and its XLR output (the website says "in phase" but conceivably that could just apply to the path between XLR input and XLR output).
In any event, though, even if it is inverting polarity I don't see why interchanging + and - at one end of the speaker cables wouldn't correct for that (assuming it matters at all, which would be recording dependent).
Also, btw, a polarity reversal would be 180 degrees, not 90.
Enjoy! Regards,
-- Al