Like most of us, if not all, I've had many a contentious discussion about cables in years gone by, so I don't really want to get into here. I will give you my opinion (which btw is NOT up for debate.
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I've tried cables from lampcord to middle-of-the-road in pricing ($200 for an 8ft pair). On many I couldn't really tell a difference, but with some they were quite telling. As such, I am a believer in cable upgrades and won't even consider a change to the offerings from monoprice. I can't speak for BJC as I've never heard them in anyone's system. I feel that the cables I am employing currently (VALAB/King Cable) are outstanding and match my speaker/amp combo quite well. Given the price of my speakers, the cables represent about a 10% investment, which I think is a good ratio.
If, or when, I upgrade my speakers to lets say something in the vicinity of $4,000/pair, would I change my cables? I'm not sure that I would, although I'd certainly be open to an audition of some higher end ones. The audible differences would have to be fairly significant though before I'd bite on a more expensive pair.