Hey, kitchen. From everything that I've read and heard, the coax cable is the better of the two choices. My guess is because it (for one thing) allows for one less filtration step.
In terms of the speaker wire interfering with the low-voltage of the subwoofer, are you asking if the speaker wire would interfere with a coax cable running through the wall? If that's the question, the answer is that a well shielded coax cable would prevent that. If you're asking if you can send a low-voltage signal over the 16-gauge speaker wire while having other high-power speaker signals going over adjacent speaker wire, then there is a possibility of interference. You could always give it a try and see if it there is enough to bother you. When I was getting ready to purchase under-carpet wires for a subwoofer, two different cable companies recommended running flat speaker wire instead of 75ohm coax for the low-level feed. I don't fully understand why, but I think that might apply to your question.
