BSW seems to be at a loss... if the 2010 Z4 base audio is like the 2010 1-series, 3-series, and X5, you have crappy 6" woofers underseat and 4 mids in the door, no tweeters, in parallel on deck power, with significant EQ curve applied to the deck power.
You need more power and you need 8" and tweeters and some de-EQing. Spend about $1200 on gear and it will sound incredibly improved. But there isn't much you can do with a bolt-in speaker upgrade - due to no power - and if you get more power, you amplify truly crappy speakers.
2007 E91 xiT
RIP avincar - check out musicarnw.com in Portland, OR.
Specializing in European car audio and entertainment upgrades done right.
Also check out OEinterface.com - OEM integration resource