hi .. I have a subwoofer and amplifier in my trunk, and strictly going by its' specs, it should be loud, but in actuality it isn't as loud as I had hoped. I was wondering ...

If I were to get an aftermarket deck with high voltage outputs (4 volts or so) would I get an increase in loudness? Currently im using a stock stereo system and a line out converter by my rear speaker, so would a aftermarket head unit with specific subwoofer control benefit me? Thanks.

It could but if the LOC was not properly installed that could cause a problem.

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LOW TONEZ AUDIO

Yes a new headunit is going to help you. Most LOC's are low voltage units unless you pay the $ for the premium units.