I know many people have added subs in the trunk. Is that instead of the subs under the seats or in addition?

Has anyone simply replaced the subs under the front seats with harder hitting subs? What size are the stock subs?

Thanks for any answers

Here you go:

https://5series.net/forums/index.php?showtopic=47747

Good luck!

For some reason i can only find the earthquake 8inch subs that can fit under there. i connected it abit different,
i used a converter to take the signal from the line to the subs then connect it to an amp, then run the speakers off the amp.
thats the quick way, but that DIY there looks more professional

I have a pair if you're interested (Earthquake SWS-8's). They drop straight in and are a direct replacement, but most will recommend adding an amp to the setup if you're going to do it. There is plenty of good information on here on how to do it all. Just do a search and you'll find everything you need. Good luck!