This would be my first proper post here. Firstly, I would like to mention for those non-believers. Cables do make a difference.!
I wouldnt say that it will turn your system in to a whole new systems with new characteristics etc. It will improve the sound in certain areas depending on the characteristics(Do some research on different materials, silver gold etc) or in some cases it will not. When listening to cables, you should listen over an extended period of time not A/B comparison over 5 seconds.
For example on a forward sounding system like naim, Laid back cables like those from Purist audio will sound wrong and unappealing. However something punchy and lively like those from JPS Labs might suit it better. These are from my experiences not any manufacturer claims and there are other manufacturers that also suits Naim not only JPS Labs.
These experiences got me started on cables and HIFI. I'm 20 now(Don't give me the you're young and inexperienced crap) and I have unconsciously been an audiophile seeking the best sound since I was 16. I got my first audiophile(Ultimate ears Super.fi 3) IEMS(In-ear Monitors) after months of savings when i was 17 and yes

it was amazing(at that time). Skipping my other headphones and gadgets in between, I upgraded to the Triple fi 10s which have 3X the amount of drivers and about 3X the price. The new IEMS were a whole new world and made me regret jumping the gun on the triple.fis too early.
Not too long after that, my neighbour exposed me to his £40k Audio Research + Sonus faber. Phew?! Best Experience I have had in my life so far. Synergy was right, Clarity was right, hell, everything was right. He mentioned that he had spent over £4k on cables. At that time i thought of it as a plain stupidity but I still tried after market cables for my IEMs and yes it did improve the sound so much that I wouldn't even consider the old cables anymore. From that experience I felt increased soundstage, separation and clarity but most importantly its synergy.
So how can you tell that you're listening to good cables? Use extended listening. Leave in on for a few days then compare, use your memory and feeling and remember the locations of your volume knob. We tend to listen to music louder to compromise the lack of everything between cables. I can write much much longer but I think I will stop for now.
I have not tried Mark Grant cables before but have bought his power cables, though I'm not his biggest fan I'm impressed with the quality of his products. I wrote this post because of the injustice I feel reading the 2 pages.