I?ve upgraded my car with Earthquake SWS-8 sub and BSW Stage1. I?m planning to by a JL Audio HD900/5 amp connect the high level signal from the HU and OEM AMP for the sub to the HD900/5. The problem is that these signals are crossover, but I don?t know how. Anyone know what frequents the HU crossover the signal between the midrange/twitter and the sub?

If possible I don?t want to use a summing device, but use the OEM crossover from the HU

You have no choice but to use a summing device. The cheapest route is getting a jl cleansweep with their summing device or getting a bitone.

There is no separate channel for the tweeters, they have a passive crossover, so there is no crossover between the midrange and treble speakers. The subwoofers are about 125 hz low pass and the front and rear speakers are about 125-160 hz high pass. The test car here is a 2008 LCI US standard audio system ( hi-fi ). These response curve graphs are with the analyzer connected directly to the head unit/ amp .

Why do I have to use a summing device? From the graphs methodsound is showing the crossover is about 125. I think that this would be ok. The Earthquake SWS-8 sould play to aroun 150 - 200 hz and the BSW Stage 1 should be able to play from 120 hz.

Thanks a look!!