is it a high quality cable? I would say since its a longer cable it has more resistance. more resistance to a speaker=less power output just like when you wire subs. the lower the resistance the more power you get from the amp. You may need a small amplifier to boost the power
+1 for this.
Use high quality cables, preferably with gold connects.
Use the same cable on both rear speakers, even if you don't need it. This will keep the signal balanced.
Adjust the front/rear volume levels to compensate. Most good amplifiers or soundcards will include adjustment for this.