Welcome!
If you use the board's search feature you will find there is a lot of discussion on this topic.
As a audio buff with a model year 2000 car (so the M Audio was not an option for me) I read these threads with interest. It seems to me that the quick answer to your question is: the M Audio gives you an improvement, but if you are not into really boomy audio and want a more refined sound you may want to consider doing some stereo work. A first step could be to replace the stock speakers- which are made out of cheap materials. This means they cant vibrate as well as better materialed speaker drivers. Youd have to find speakers that would pare up easily with the stock amplification. Once you move beyond simply replacing speakers, you get a lot more complicated and I would suggest if you want to go this route that:
1- find a reputable place- the aftermarket stereo business is sooooo corrput. also if you are not known to the people there they will yes you to death and sell you everything you dont need.
2- dont let them take the nav system/stock head unit out of the loop. you need it.
3- if you are leasing be careful as to the modifications you make
4- think carefully about whether its worth it to make these mods.
good luck on your M5 quest
ME
M sound system...do you suggest this option?
Linear Mode
