Wow, I bet I could make some tin cans that have more definition than those OEM pieces of crap. Haven't really looked at pictures before.
The OEM crossovers and cutoffs are weird. I have no doubt that you'll find the audio a lot better if you install a more intelligent amp, but I don't think what you have in mind will work unless you have a separate cutoff that turns off the midrange, otherwise the sub will be trying to make high frequency noise and will sound awful.
'99 528i ('98/12 build). 282,000km BY29428/Royalrot
Breaking My Wallet since 2009
Mods: Stoptech SS brake hose, 280piece toolkit resting on trunk floor, Beisan VANOS seals, increasing amounts of duct tape and glue
'99 540i (grandfather's)