subwoofers

I bought a (2) splitter for this as well, worked fine. 

What is the model of subwoofer?

The answer is with a simple RCA splitter or crossover.

Some subs used to include high pass filters (part of a crossover) in them.  I encourage you to get some measurement gear like OmniMic or Room EQ Wizard to help you integrate your main speaker with your sub. 

Hsu speakers sells custom frequency crossovers, but miniDSP will also work great.

If the preamp has a tape out or a second set of outputs, you could use those, provided that your sub has a separate volume control.