A difference of 8 feet will not make an audible difference. This is easily shown with the math, but without the boring details:
Assuming the speakers have a 90 dB/2.83V-m sensitivity at an 8-ohm nominal impedance (1 watt) and that the speaker cables are 14 ft and 6 ft of #12 copper, the voltage drop to the furthest speaker will result in -0.035 dB. If you're 8-feet away from the speakers, you can compensate for this by moving that speaker a quarter of an inch forward. I didn't take capacitance and inductance into account - I don't think it will make a difference, but I could be proven wrong.
Assuming the speakers have a 90 dB/2.83V-m sensitivity at an 8-ohm nominal impedance (1 watt) and that the speaker cables are 14 ft and 6 ft of #12 copper, the voltage drop to the furthest speaker will result in -0.035 dB. If you're 8-feet away from the speakers, you can compensate for this by moving that speaker a quarter of an inch forward. I didn't take capacitance and inductance into account - I don't think it will make a difference, but I could be proven wrong.