I just installled my headunit like normal with the amp still there (16y/o at the time, didnt know better haha) and it still sounded fine. It really depends what you're doing. Are you adding a low range speaker or just running the door/deck speakers? If you're not going to add a sub or another amp then I would definitely wire the factory one in. 4 6.5''s on a headunit sounds like an alarm clock radio IMO.