I think it depends on how good your mids or mid-bass drivers are. If your mids put good mid bass then 65hz isn't too low at all. If they don't then they need your subs help for mid bass and you should crossover at 80hz or even 125hz. But of course this all depends on your cars interior acoustic characteristics. Good Luck.
What kind of crossover are you using? Is it the internal crossover from the amp? If it is try running it a little higher. Of course this is useless, 'cuz I don't know what you are running.