Speaker Cable Length

I believe you should look into Iconoclast speaker cabling.  Really good science behind them.  They have been my final cables for both speaker (8-ft) and my 5.5-meter RCA interconnects from preamp to amp.  

I use 5-meter Klee Grand Illusion. No problem. 

I will also give a shout out to Iconoclast.  Bob Howard at Iconoclast is great to work with if you have specific issues.  Bob wants me to try their speaker cables, but I am all set in that regard.  I am using their power cords.  Best manufacturing processes and quality build.  Great at dealing with problems if you have them. The specs on their cables are available.  The chief engineer at Iconoclast is Galen Gareis, who has as much experience and expertise in this area as anybody.  No snake oil at Iconoclast.

It's not about our opinions, it's about science. Plain old OFC is the best thing you can use. Refer here for gauge:

http://www.roger-russell.com/wire/wire.htm#wiretable

If it was me, place the amp between your speakers in the middle on an amp stand. Your 8’ cables should work then. If you are moving the rest of your equipment off to the side, you can run a longer pair of balanced interconnects to the amp. 
The rule was to use the shortest speaker cable as possible and run the longer length to the amp using balanced xlr’s. Having an amp low on the floor will not hurt the sq.