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One option worth considering would be Transparent The Wall Reference speaker cable. These are designed for in-wall and longer lengths.
The Reference in-cable itself is pretty reasonably priced. Where things can get expensive is with the termination bricks, which are specifically calibrated to the length of your cables.
Transparent has varying grades of bricks, so you can attach the more moderately priced "High Performance" or "Premium" bricks to the Reference cable.
I also run 18 feet speaker cables for "girlfriend approval factor" reasons. Essentially, the gear is tucked into a corner nook. The speaker cables run along the back wall behind a couch, with the speakers on either side of the couch and facing into the room.
I was initially running Mogami 3103 and later upgraded to AQ Bedrocks. The Bedrocks are attached to Harbeth Compact 7es3s speakers and an Akurate 2200 amp. I've also used the Bedrocks with an Ayre AX-7e to good affect. I almost upgraded to the transparent The Wall References, but instead chose to save the cash and just stay with the Bedrocks for the time being.
Hope that helps.