Speaker cable length

I also have a similar set up, but keep shorter speaker cables (6)', amp resides between speakers, and running longer ICs( 15') from amp to source.
Hope this helps.

Problem is that between where my speakers should be positioned for best listening is a large floor to ceiling window so putting my amp between speakers wouldn't work.

Depends on the cables and amp.

At CES in January 2007, AJ Conti of Basis Audio ran a pair of speakers with his cables. One speaker was wired with a 6' pair and the other speaker was wired with a 75' pair.

Not a single listener could discern the difference.

Go figure.

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.

There have been many posts regarding speaker vs IC lengths. Many opinions and not much science. The one fact is that the time difference between 27 feet and 18 feet of speaker cable wil roughly translate into a few millionths of a second. Your ears are probably better than mine but I couldn't hear any difference. Don't worry about it. I chose to run long shielded ICs to monos sitting beside my speakers, but that is just personal preference. Heavy gauge speaker wire lessens the resistance, so try 10 or 12 gauge. One very affordable option is TO buy Canare 4S11 4 wire speaker cable. It is effectively 11 gauge, twisted to reduce RFI and EMI, unshielded with good copper, and roughly $1.00 per foot unterminated.