I need advice

I recently installed a 300 watt amp with 8 gauge power and ground wires under my left driver seat. The amp was not strong enough for what I needed, so I got a 500 watt amp and am planning on installing it. I need to run 4 gauge wire to that amp, but I don't think there is enought room under my left driver seat to fit the amp, leaving me the option to install it under the right passenger seat, that will leave me with interconnects, remote wire, ground and power run to the left side not being used. I have a 4 channel around 200 watt amp I could put there and run my highs of it or replace my factory deck with a 53 by 4 watt deck and run my highs off that. What route do you suggest I take and if I replace the deck and use that power should I replace the factory speaker wire. Sorry for the length, but any suggestions would be appreciated.

What has led you to believe that your amp is not enough?  What are you trying to do?  The difference between 500 watts and 300 watts is not a lot.  If you have the 4 channel, then I suggest you use it.  Yes, you can use the factory speaker wires.

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My ex once told me I have a perfect face for radio.

Geepherder is right on. The difference between 300 and 500 watts is less than 3 db! The question of watts gets into the game when you talk about headroom. That's a function of type of material, AVERAGE level and peaks. So, how loud do yo play it usually and what kind of material are you listening to?

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Tom