Your biggest enemy with long speaker runs is resistance, so if you're stuck with 27' runs, go for the heaviest gauge cable you can find (and is practical).
If you're concerned about the different lengths (27' versus 18'), then just use two 27 foot lengths and hide the extra few feet of cable behind your amp or speaker (but don't coil it up).
That said, at the speed the signal travels through the cables, you're unlikely to hear any difference at all even if you use different lengths of cable.
If you're concerned about the different lengths (27' versus 18'), then just use two 27 foot lengths and hide the extra few feet of cable behind your amp or speaker (but don't coil it up).
That said, at the speed the signal travels through the cables, you're unlikely to hear any difference at all even if you use different lengths of cable.