Newb question: are there any drawbacks to using an RCA to 1/8" adapter on a receiver's sub pre-out, and then using one of these...
http://www.ramelectronics.net/assets/images/180-3-5S-RCAPX2.jpg
to connect to the subwoofer? Would it make a difference if the above cable is stereo or mono?

Can't think of any.

If the original signal is mono, only one of the RCA jacks at the end of the line will be carrying signal. No real downsides to doing this, just make sure the connections are nice and tight.