Does the length of the Cable matter?

All cables are tone control by nature, longer cables will have more effect. Maybe the first cable you will try will not be satisfactory. A shielded cable may affect the top end from its capacitance and at this length an unshielded cable may pickup hum. Also the dielectric of the cable degrades the low level texture of the music, accumulating over the length of the cable.

To maximize the SQ at that length, a DIY approached would be to build and experiment with RCAs  assembled from a pair of spaced cotton covered bare wire, or with a central the cotton wire with layers of further cotton dielectric sleeve to get a diameter of 3/8 and fish the assembly in a 3/8 tin copper braid to get a shielded RCA of lowed capacitance.

cleeds

thanks for the clarification. Is the construction of a balanced cable more resistant to interference than a typically shielded rca cable, that was my assumption to suggest the conversion.

Over the years, I've used long and inexpensive rca cables, even 25lf to reach a sub, with no hum or problems, so I think OP should be ok. 

Properly grounded and shielded, and you'll be fine at 15 ft. 

Big mistake will kill sound quality.