How to hook sub to 2 channel syhstem

guys since my preamp is single ended and I can only choose one input at a time - How would I hook up a separate amp to run my subwoofer in 2-channel mode except by getting a splitterGuys since my preamp is single ended and I can only choose one and split the Front left/Right channels- thanks
Get an REL. They run off the speaker taps. No splitters needed, no crossover, won't hurt a thing.
I 2nd the REL. Uses the SPKON cable direct from amp taps. Buy a upgraded cable from much better than stock.
Vandersteen 2Wq works great with systems like yours (separate pre and power amps). An included crossover box goes between the pre/power amps to roll off the low end that goes to the power amp. Then a set of wires run off the speaker taps of the power amp to the sub. The sub is NOT using any of the power from the amp (it has a built in amp) but instead senses the signal and that's what drives the sub. Essentially , the sub become integrated with the speakers "better" because it is sees the signal as the speakers see the signal (power amp output). This also makes your powers amp work less since the roll off prevents the amp from pushing watts for the low end.