There isn't really anything 'upgraded' about them by having the HK badges on - as I think HK did all the audio for the P38's, but the 'mid line' and 'high line (which I guess you have if you've got a subwoofer and cd-stacker) means that instead of the head unit having amplifiers that drive the door speakers, each door has an amplifier in it, and the head unit just sends a balanced line level signal to each door amp.
The good news... the amplifiers in the doors sound really nice, and are far better than what a head unit amplifier would do. The bad news is that you can't just rip out the factory stereo and put a new head unit in and have it work.
You can however use the amplified outputs from a new head unit and feed them into some attenuator circuits which the information about is here:
Stereo attenuator circuits
Yes, the link it to the 'other' site, but it's the main website part - not the forum..
The attenuators are cheap to make, and bring the amplified outputs from a replacement head unit down to the level expected to be input on the door amplifiers that you will have.
The subwoofer (if fitted)... most people just put an RCA plug on it and plug it into the line out subwoofer feed from an aftermarket head unit - but sometimes the level isn't high enough to get decent bass... it isn't on my P38, so I put a booster in line with it, which is fine, except I get a pop from it when it powers up/turns off - so I might look at either a) adding something else to filter that out, or b) fit an aftermarket subwoofer amp which is designed to take a lower line-level input.
Hope this helps..
Marty
Edit - Gilbertd typed faster than me... but SNAP at linking to the same page...
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