Hi Jules,
I've never noticed any sonic issues with long or short sub cables; the propagation speed of an electrical signal through cable is around 40-70% the speed of light, depending on frequency and cable material, and at the very sluggish frequencies of audio it shouldn't make any difference.
For high audio frequencies, up around 5KHz and beyond, there are arguments about 'skin' effect, where the signal travels along an annular ring at the outside of the conductor, but comms engineers tell us this only happens beyond 1MHz. Along with EMI, it's VERY important in the design of motherboards for PCs, but not too important at audio.
Cheers,
Hugh