The stock E9x system without logic7 is a very underpowered system as proven by the many people complaining about the sound. There is a distinct lack of bass, especially midbass in the saloons. I have personally replaced my underseat subs with Infinity 6.5" mid bass driver that is now powered by an amp in the boot and boy is there a marked improvement in the mid-bass and as stated you can actually feel the music now!
Word of warning, if you dont have a logic7 system, dont try and amplify the stock underseat subs as they are 2ohm to work more efficient on the low power from the head unit. I amplified my stock subs and consequently melted the coils on both subs, hence the replacement :-)