Power cord and sub cable

I was moving some furniture around last weekend and I noticed that the subwoofer cable (SVS brand, shielded) that feeds the sub in the back of the room, had been laying right on top of the power cord for the sub. I wasn't aware that there was a problem before, but just for my peace-of-mind, I seperated the two. There does seem to be a difference in the sound, but this is hard to tell with a sub.

So my question is, how far does the cable need to be from the power cord? I don't have a lot of room so is a few inches enough?

That depends on the effectiveness of the "shielding" of the subwoofer cable ... if it's as good as it should be (and I have no reason to think it's not), then a couple of inches should be more than adequate.