I'd go 85%! ;-) The XV-1S is a very good cartridge. I've never heard one do anything wrong, in Dan's system, ours or elsewhere.
David,
The ZYX pre-pre has a fixed input impedance of 125 ohms, right in the heart of the optimal range for a 4 ohm MC when using an active gain stage. It's perfect for a LO ZYX (naturally) and should be good for an XV-1S too. Remember, getting impedance precisely right is far less critical with an active gain stage than with SUT's. We play at ~200 ohms and anything from 100-200 would be fine. Close counts.
Like an SUT, the pre-pre provides no RIAA equalization. It's intended to plug into the MM inputs of a phono stage.
Your SUT's provide 26 db of gain. The ZYX pre-pre provides only 20. This may or may not be sufficient, depending on the gain structure of the rest of your system. The gain calculator at www.kabusa.com may help you figure that out.