Originally posted by xivera
Although you have already set the crossover point at 80 Hz at your head unit, setting your EQ low pass crossover at 90 Hz should be good. (since frequencies above 80 Hz is already being attenuated).
A good rule of thumb is to set a low pass crossover to the lowest frequency your front stage speakers can handle without distorting the rest.
Usually, the lowest frequency your front stage speakers should be your low pass frequency (or around that value) without muddying the higher tones. This is to keep the less omni-directional bass tones from pulling the sound stage towards the rear to some degree. (Remember, the higher the sound tone, the more directional it becomes. ie. voices vs. "quad" bass)
This is for my subs only.