You should be able to use the main 596 o/p direct to the 568 and the aux o/p via the 518 - at least that is what I have done with my 598/561. Makes an A/B comparison easier.

HH1 Photo 34. 598DP, 504, 518, 561(M), DSP5000, DSP33C, DSP33
2nd Room: 541, 555