I have drive two weeks new for me e61 530xd. Car has business stereosystem. Biggest failure of this stereo is in lower voice output. Even little bass put lower units cracks. I bought Blaupunkt thb-200 active sub last summer to my boat. Sub sounds good in boat but its form dosnt match very good in boat cabin so i think i put this to e61 and buy different in boat next summer.

This install isnt very professional made but it done its work.

I put thb-200 to cars trunk under the front hood.
Sub is under the hood and all space in front are still usable.

You dont need to use line input to get the control signal to the sub. Thb200 has possible to take input signal from other speakers wires.
I take passenger seat out (lift forward) and take out under seat orginal sub cover.
After subcover is out there can see the speakers wires. I connect thb200 speaker-input-signal wires to
under seat orginal sub-speakers wires (first i measure wich speaker wire has + and wich -).
After connecting thb200 input signal wires to under seat sub i connect thb200 + wire to the battery (+ wire much have 10A fuse) and - wire to cars body. Thats all connections what you need to do.

In pics can see that i skrew sub throught frameworks (where thb200 lie down) to plastic part (this plastic
part makes frame more stolid). I also make other plastic install to up from spare wheel base.
Now when you close the upper hood sub lie down and are connected to spare wheel base bottom (picture with red arrows).
This is important, otherwise the sub start to move when use it and bass cracks.

I turn sub crossover ~80khz and car bass control in idrive in middle and it works.
I think this succeed very well and i'v been happy, no more bass cracks and thb200 gives out more
than i expect
In e61 business stereo has speaker in doors, under seats and front roof, its not so bad system if you
put more power to lower voices. Iam not nothing golden-ear so i think this is enought to me.
Thb200 costs 150eur in here so this is quite cheap way to make some uppgrade