Impedance isn't the only issue but gain (output voltage) is also important of course.
If your pre-amp has an output impedance of 50K and the Polk sub has an input impedance of 10K Ohms, you may be in trouble particularly if you use long interconnects. The voltage across the sub may be too low to drive it.
So try to use a short interconnect between the sub and the pre-amp outs to see if it's any better. If it is, then your pre-amp output impedance is too high. It's probably not too high to drive most power amplifiers but it may be too high to drive the sub.
Also try turning up the volume of the pre-amp all the way to see if it makes a difference. If it does, then gain/output impedance is at fault.
