Nearly nothing, nothing measurable at all. Your audio amp will never carry as much current as your jumper cables, and #6 jumper cables will carry the massive current it takes to start your car when the battery's nearly dead. If the power wire feeding your audio amp is larger than the wire running from your alternator to the battery to POWER that audio amp, then it's wasted copper - you can't pull more power than your alternator can supply without gradually draining your battery while you drive. Further, that one alternator wire carries power not only for your amp, but also for the headlights, taillights, heater blower, wiper motor, and ignition... plus any other extras your car might have (electric windows, etc). That ONE wire carries it all, so there's no need for anything bigger than that one wire.
I'm betting your alternator wire is a #8.