HK system and aftermarket amps

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My buddies E92 with HK sounds awesome so when I was shopping for the X5 I made sure to get the HK system thinking it would sound awesome as well, but the bass response is lacking when parked and non-existent when driving. So I added a 12w3 and JL 500/1 amp. I searched and searched for ways to add amps to the HK system and ended up following a couple posts here that ran a LC2i off the OEM amp sub output. It works, but the freq response is lacking range. Hits those super low freqs, but only booming lows, no punch or even close to mid response. So then I tried running off the rear speaker outputs, but because the rear speakers are filtered then it hits mid-range freqs but no low freqs. I've tried tuning the LC2i but its just not where it should be if it werent going through the OEM amp first. I've had this sub and amp in a few vehicles and know what it s capable of producing, but the X5 is proving to be a challenge.

Are there any options other than the above for running an additional sub amp that doesnt get limited by the filter of the OEM HK amp? I saw that Bimmer-tech has an amp with what looks like RCA pre-outs, but its not compatible with the S688A HK system.