Is it better to use the HPF/LPF on provided on the amps or on the headunit or both at the same time?

It really depends on what amp or HU and what level your prepared to go to in setting it up. usually your better off using one or the other, as you really need to use an RTA to use both, as you cant rely on the markings to set them, when your using both.

The advantage of using HU is you can hear what the adjustments do as you make them and so its often easier.

peace
Cyberpunky

If you filter using both,consider them summing to cause a new filter function. Two 12db/octave filters in a row with the same cutoff = 24db/octave filter.

as long as you have an RTA to set them up as the indicated filter point may not be correct and they only sum if matched to the same freq
peace
Cyberpunky

i like to use the ones in my hu, only so you can adjust from teh driver's seat

- Stooge007 out

i don't think it realy matters in the end.
the ones on the head unit end up being easier to change in the long run.

and it makes it easier to hear the differences that you are applying as you don't have to run around the car to the boot to change settings and then run back to the driver's seat. That makes it heaps easier to assess what differences the different slopes of filtering make.

i have my h/u xovers set at 80khz and my amp set at 45 i think it is....this is just to experiment around for a while. the best benifit i have found so far is that when playing nice music i can turn off the h/u xover and have the fronts play down to the lower x-over but for techno etc where i want loud volume i turn the h/u xover on.

can anyone see any damage issues here?

no

me thinks you meant 80 Hz...80 Khz may be a tad high lol
peace
Cyberpunky