Speaker cable length.

There’s a lot going on inside those cables besides just getting electrical energy from Point A to Point B at a fraction of the speed of light. The insulation (dielectric) is not electrically inert, which is to say that it acts as a capacitor absorbing, then releasing energy slightly out of phase with the main signal. So, 5x the length will increase this by a factor of 5, and the cables will sound different.

Whether this effect, and other factors, will be audible to this listener in this space with this system is difficult to predict. As some have suggested, and blind A-B comparison may put your mind at rest.

@n80 

many audiophiles would say they could hear the difference

What they say and what they can blindly ID are usually polar opposites.

I had contemplated this issue several years ago when I had to do the exact same thing for my speaker wire run in my living room.  Left speaker is a 20’ run and the right speaker is 55’. Both cables are Canare 4s11.   I considered running a loop for the left speaker but in the end I simply ran the length that was needed as my determination was that the voltage travels at the speed of light so how big of a difference could there really be?  Well, I’ve been dealing with a sound imbalance ever since.  I’ve tried swapping out the speakers and swapping out the amps just to eliminate that as a possibility for the difference.  I’ve tried multiple toe in adjustments and speakers configurations.  The best solution I came up with was to put a potted plant in front of the left speaker to help balance it out and it does work ok.  My preamp does not have a balance control so this was the best I could do.  TBH, it wasn’t until this morning and reading this thread that the one thing that I didn’t think of as a possibility was the difference in cable lengths for the speakers.  As of now I am of the belief that the cable length can make a difference.  My cables are run down through the floor and through a soffit in the basement ceiling that contains other runs of cabling and power wires so there are other variables as well.  I do cross power at 90 degrees and am conscientious of my cabling runs.  This system is mostly used for background listening and fills the room just fine but it is a struggle to get good sound.  If this helps the OP in some way, please learn from my mistake as there’s no easy way to fix it now.  

Long vs Short Speaker Cables: Does it Really Matter? | Audioholics

I have an 8' run and a 17' running under the floor similar to your config.

Imagining is spot on centered, volume  seems the same level in both.

Cables are Morrow SP4 or SP5 can't remember. Amp is Hegel H20, speakers Magico A3.