Really ? I've never seen anyone use mains cable for speaker cable
Bottom line is trust your ears...if the salesman says 'this is the best amp in the world' but it makes your favourite CD sound like a pile of

to your ears, don't buy it.
All I can say is don't dismiss anything (perhaps with the exception of drawing round the edge of CD's with a green marker

) until you've tried it and listened to it.
I was fortunate enough to have a 'proper' hifi shop near me that leant me a selection of speaker and internet cables over a weekend. Without spending crazy money, £8.00 / metre speaker cable and £75.00 for a pair of interconnects, there was a subtle but noticeable improvement in the sound.
Bottom line is, you wouldn't put a set of remoulds on a Ferrari, why would you use bell wire on a hifi you've just spent, £500, £5000, £50,000 on ?