I'm always sceptical about AV cables in general and particularly about speaker cables. Cables that are (effectively) putting a significant capacitance in parallel with the speaker could have the effect of muting the treble, but I think you'd have to try quite hard to find a cable with enough capacitance to actually make an audible difference.
In my experience there are only three significant factors with speaker cables:
1) Don't ever coil them. If they're too long, then concertina them.
2) It's good to keep them well away from mains cables and from each other to avoid interference. If you need cables to cross over each other, make them cross over at right angles, don't lay them in parallel.
3) Far and away the most important issue is that, at any frequency, the impedance of the cable must be a small fraction of the impedance of the speaker. So, for example, my Nautilus 803 speakers are nominally 8 ohms, but can go as low as 3 ohms at certain frequencies. I like to keep my cables to below 5% of speaker impedance, so the total impedance of the wire has to be <0.15 ohms (and I usually aim for <0.1 ohms for safety). With speaker cables it's usually only the resistance that makes a significant contribution to impedance, so if the cable has a resistance of <0.1 ohms then it's going to be fine for 803s.
The only time I've ever heard speaker cable make a difference to the sound of a speaker is if the resistance is too high. What happens then is that the percentage of the amplifier output that is expended in the speaker rather than dissipated in the cable can vary detectably with frequency, which affects the speaker's overall frequency response.
Two (verified) anecdotes illustrate the point nicely. In the first a cable-sceptic invited an audience who believed in the virtue of expensive cables to come to a demo. He played a piece of music using cheap-looking speaker cables. Then he made a big show of switching the cables over to these huge, expensive looking things, almost an inch thick, and played the music again. Everyone in the audience said things like "oh, that's so much better, oh please tell me you can hear the difference there, it's night and day!"
It turned out that actually both sets of cables were dummies, and that the wires actually connecting the speakers to the amp were identical in both cases and hadn't been touched. (There are, of course, a lot of reasons why a system might sound better as a result of cables being changed that have nothing at all to do with the "quality" of the cable - placebo effect is just one. Other possibilities include a new cable not suffering from corrosion at the contacts while an old one does.)
The other story involves an audio exhibition a couple of years ago, where a British speaker manufacturer named Quad wanted to show off some of their new high-end speakers. To demonstrate them they hired a guy who was a sound engineer: his job was mixing music tracks to produce the final recording. He was very well-known, and well respected: had won a number of the most prestigious awards in the music industry, so he was about as "golden-eared" as it's possible to get.
One journalist noticed, during the demo, that the speaker cables looked a bit unusual - they were a bright orange colour. So he asked about them, and it turned out that they were actually mains extension leads of the type normally sold for use with Black & Decker hedge trimmers. So, out of all the possible types of speaker cable this supremely talented sound engineer could have chosen, he'd actually chosen mains flex - because, in his opinion, that sounded as good as anything else does.
