Ideally you would run the sub of a pre-amp. You could use one of the ZA 3's sub outs of course but then you will miss a channel (low frequency is usually on both channels so it shouldn't matter much). Or you could use the speaker level inputs so from Both ZA3's into the sub and then to your speakers. The advantage of that is that your sub can filter the lows the sub handles from the signal so your speakers will have cleaner mid range. Disadvantage is much more speaker cable is needed.