Sound is always cleaner going through RCA vs the typical "powered by deck" amplification. Your deck is at 6.5v preamp output for RCA's. That is very high and way above the average. Its not your deck being an issue, its that you aren't really connecting to an "amplifier". I think the factory amp requires higher voltage. I think they did this in the event someone hooked up an aftermarket radio. If the voltage requirement was only 4v and someone hooked up their 40x4 aftermarket deck, it would fry the amp or just clip out.