For your subwoofer, you will most likely have to get a couple RCA splitter cables like the following:
You would connect the splitter cables to the right/left RCA outputs on your preamp. The splitters allow you to further connect separate left/right RCA cables to the amp (for main speakers) and to the subwoofer inputs.
Sometimes you can get an amp that has 2 sets of RCA outputs. The Vincent has two sets. On your Onkyo preamp, you can use the single RCA subwoofer output on the back/top for the Klipsch subwoofer. Just connect it to either the left or right input on the sub.
NOTE: Even though the Schiit preamps have 2 sets of outputs, you can only use one at a time.
The Parasound P5 actually has an analog crossover built in so that you can split the frequencies between MAIN RCA outputs as separate SUB RCA outputs.