I have tried a variety of thick cables for a variety of passive speakers, especially for long runs, from as cheap as power extension cord to moderately expensive Monster Cable.
Honestly, I simply can't hear the difference, which is maybe just a reflection that I don't have "golden ears" or that my other components are robust enough that special cables are simply not needed. (Some might argue that my other components are of too poor quality, bad match etc. to benefit from better cables).
In any case, my Cable experience, poor ears, wrong components or otherwise, should not disuade others from trying special cables....if they work for you, as many claim, then go for it! To each his own and good luck!
This next bit may sound funny: Actually, I do use the thick Monster Cable over the thick power extension cord for the passive speakers I use...even if I admit to being unable to hear a difference. I think of this as an insurance attitude: Better for me to make sure that cheap cables are not possibly degrading the sound in some horrible way that I have not readily perceived than to use cheap extension cord, especially seeing as I already paid for more expensive Monster Cable anyway! And, of course, for the additional cost, the cables look significantly more professional than ugly bright orange extension cord but, *sigh*, I would certainly prefer to be able to hear an improvement!