E53 Speaker Passive Crossover?

[trhaverstock]

My 2004 E53 came with a Pioneer head unit and the factory DSP and amps all removed. I have no idea what OEM system this had, but it's a 3.0 M54 with a 6 speed manual (who-hoo). The Pioneer sounded just ok but looked bad in my dash so I replaced it with an Xtrons OEM look head unit and replaced all 6 front speakers at a later time (dash and door) due to terrible sound and age :-). I went mid grade on all. Of those speakers, I recently dropped 6.5" coax speakers in the front doors and it appears there is a passive crossover somewhere upstream that is not pushing a full signal because my tweeters in there are not tweeting and I just have the midrange sound. Any ideas on where I can look? My concern is that something is tuning to the OEM head and speakers giving me a muddy/less than optimal sound overall. I searched the web and found some scant info on the X3s from this area but not a solid lead on the E53. Thanks!