preamp with only 1 set of pre-outs: adding sub....

A pair y connectors will allow you to connect both to the outputs. Not a purist arrangement but works fine and I doubt anyone can hear the difference.

I connect my sub using high level inputs connected to my power amps. I find it far better. But if you want to use pre-outs, the y connector is the way to go.

ACI Speaker Level Converters (scroll down the page)...these connect to your amp's speaker outputs. They work great.

I have a powered sub. I ran my pre-outs from the pre-amp to the high pass filter on the sub and from the outputs on the sub back to my amp and ran my speakers from my amp, using the crossover on my sub.