We upgraded the sound system on this W205 C300 with an amplifier and subwoofer. This is a non-Burmester car which is distinguished by its black speaker grills and lack of tweeters in the tweeter opening on the front doors. Regardless, the Burmester system still offers no improvement in bass as they still only get the small 3.5" door speakers and no decent sized woofers for good bass reproduction.
On this W205, we installed a Pioneer GM-DX871 which has gold platted input terminals capable of high level speaker inputs. This amplifier is good for 500W RMS at 2 ohms which is a decent amount of power for a sedan like this. We mounted the amplifier under the trunk floor so it's completely out of sight. There is limited space under the front seats and a larger amplifier like this likely would not fit. Another bonus with the GM-DX871 is the remote knob which we mounted near the driver's side light controls so the driver can adjust the gain on the fly.
The subwoofer and enclosure we chose was the Alpine Type-R 8" sub with the matching Bassworx Alpine Type R enclosure. The black carpeted enclosure sits neatly in one corner of the trunk and with the banana plugs, our customer can easily remove the sub if he needs to move larger cargo.