high pass filter on speaker.

hey 12v. well i got a accord with a decent panasonic deck. I got an orion sub with fosgate amp. there is also a blaupunkt 4ch amp. Im using stock speakers for now but plan to upgrade. The crappy thing is my deck doesnt have a HPF built in. I turned the bass to -12 (max) but there are still low ranges hittin my speaker, I was planning to use 500uf cap in series to block it out. But what frequency should i cut it out at so it sounds good ?

should i hpf it at 80hz or 120hz. btw the speakers are 4ohms.

thanks for any input.