Need recommendations on 15ft speaker cables

Townshend Audio F-1 Fractals.

I have a pair and they are excellent. And the way they treat their copper wires, the length doesn't matter. Nor if 1 cable length is different from the other. 

It doesn't matter as long as they aren't 20 gauge or smaller. All speaker wires sound the same. Save your money and buy more music.

Transparent cables are extremely, well, transparent. I own Transparent. Only the highest levels within the Transparent lines is tuned to specific components. They are outstanding in systems where you want the wires to get out of the way. What are your components? If your components are only just adequate, then they can reveal things best covered up. I highly recommend them for audiophile systems.

On the other hand Cardas wires are built to be more forgiving at the low end and more transparent and detailed at the high end. So, typically these will be a great match for less than top of the line audiophile components.

Mogami.  They make a quad conductor speaker cable which add up to about 10 gauge (it's metric so not exactly).  You can find the 2 conductor at Parts Express and I think the 4 conductor at Markertek. 

My advice is to buy a high quality but affordable cable set to use as a benchmark before spending big money.  Erik mentioned Mogami, and Canare is another, both available from World's Best Cables on Amazon, see: Canare 4S11 or Mogami 3082

Once you are accustomed to these, you can try borrowing some Cardas, Audioquest, Nordost, Transparent or other higher priced cables.  It will help you gain perspective on the value of spending more, and we'd love to hear the outcome once you've tried a few.