Hey team i got half way through this retrofit before i thought of posting so bear with me lol.  the story: Ive done this retrofit before with my old e92 and when i did i fitted the amp before the speakers and the amp alone made a huge difference to audio quality and the 535d was in dire need of better audio.  Unlike the hifi option in the e92 the e61 hifi system doesnt have a amp so the only difference between standard and "hifi" is the addition of subs and a small sub only amp.