Cambridge 640c output voltage?

I'm looking into buying a passive preamp for my system, and the manufacturer is asking for the output voltage of my cd player. For the life of me, I can't find it. I've looked in the manual, on the website, and even emailed the U.S. distributor, to no avail. Does anyone know this info or how I can find it? Thanks in advance.
I have used the 640c with a Passive Pre-Amp (FT Audio's LW1S2) with no difficulties, so the output is likely north of 2.5v.