Reply to Pre-listening problem on 2 synced up S4's?

Hey guys,

The last couple of weeks we have tried to sync up 2 S4's with midi. We got this thing kind of working, the only problem is pre-listening a playing track on the Slave S4.

On the master S4 we got 1 RCA to RCA cable running from the Main Out of the S4 to the amplifier (the main sound source) and 1 1/4 to RCA cable from the Main Out running back to the Slave S4's input channel C.
We did this so the Slave S4 user could pre-listen and cue up his tracks to the Main Out signal. The Slave S4 receives the main out sound on channel C but when pre-listening to this channel C the sound is delayed and behind on the original sound on the speakers, which makes it impossible to mix in on time.

Is there anybody who knows why the signal from the master S4 to slave S4 is delayed and how we can solve this?