Im no expert but what I did to my '04 coupe was to mount my aftermarket HU under the dash in the cd holder space. It was a clean install and still leaves a small space for storage.

For a while I was running line outputs from the rear speakers, it was ok but once I got the alpine deck the sound quality was WAY better . But since the accord comes with a amp it wasnt as loud but if you remove the bass from your in-car speakers(with crossovers)and replace it with sub bass it becomes unnoticable.