replace the BOSE. Look into Morel or DLS since your pockets are fairly deep. You shouldn't be disappointed. Maybe you'll get some more advice in a second
Amp? You can try using the stock amp, if you're happy with the sound & output more power to you. A new amp should not disappoint you either. then again i dont know how the mercedes amps are setup.
You should hear a very significant difference compared to stock stereo, especially if you set it up correctly.
carPC to a mercedes, i cant help you there wish i could though.