adding subwoofer with no pre amp source

It seems this would be a common question, but I couldn't find it on the forums. The setup I want to install is in a BMW E39. I'm keeping the factory head unit (stock 4v Pre-Amp) and changing out the amp. I want to add an additional amp for the sub, but the head unit doesn’t offer a Pre-amp for it and the amp I’m putting in doesn’t have a output for an additional amp either. Normally I would just tap into either the front or rear preamps to get the source there, but I’m worried that it would decrease the level inputs going into the amps if I have it run into two sources. Also I hate the idea of the sub fading out if I was to for some strange reason fade the sound to the front or back or balance it to left of right. Is there something I can get out there that would allow me to get a sub out from all of my preamp sources? I'm sort of surprised something doesn’t exist when you got allot of useless junk out there being manufactured. Thanks for the input guys! --=Keith

Well what if I got an electronic cross over? That would accept a 4 channel input and give me a sub out right? Would this effect the preamps? I believe this should be okay for the amp to pick up the signal right? Thanks! --=Keith