Something new - Since this has turned into the master OEM+ upgrade thread for speakers, I'm now theorizing about the OE DSP woofers in the back, I don't have exact measurements but I know they are around 5-5.5" or so - I need to take some measurements when I have my car back - From researching the E39 boards and knowing we basically share amps, looks like each woofer gets about 20-40 watts RMS from an 8ohm load, and is dual voice coil.
Its been a while for me, but depending on how its wired, you could potentially (once I check measurements) use the 4ohm or 8ohm versions of these Dayton Audio 125 Reference woofers - they are efficient (87 db for 8ohm, and 89 db for 4 ohm), and the frequency range is almost OEM (65-5400 hz on Dayton's, 30-6000 hz on OEM)
Dayton Audio RS125-8 5" Reference Woofer
Wondering how I can make an OEM+ woofer upgrade and keep the DSP amp happy and healthy -
Only question is how would wire them without taxing the amp - if you wired both + and both - together on the 8ohm version would that be a total 4 ohm or 2 ohm load?