How do you use a sub with active speakers

I feel dumb about this one. I'm playing with a pair of Quad 12L Actives. I normally connect my REL sub using the high-level method from the amp's speaker terminals, but the amp is no longer in the system. The Quads are connected from the preamp's balanced outs. Do I use the preamp's unbalanced outs, via a Y adapter, to feed a mono preamp signal to the REL's low-level input? Are there other options?