Looks like the Zpre has 2 sets of outputs. Just use the 2nd output. You don't need a specific "sub out." Those are normally reserved for A/V processors which will send a specific sub signal to the sub output, if setup as such.
All you need to do is run a set of RCA's cables from the 2nd output to the sub, then adjust the crossover, volume level, and possibly the phase, on the sub to your needs.