There are two approaches, fit a DSP amp purpose made for the 166 or the after market amp
+ original, sounds great, extra equalizer goodies found in menu, plug and play
- PITA to fit as have to take out rear seat, rare, can be cheap or expensive on ebay, need to fit bass speakers in parcel shelf, needs wiring loom addition to work
+ cheap, lots of options, uses cars line out, widely available
- variable amps lead to varying performance, probably still need those additional parcel shelf speakers, not a lot of room to fit behind dash, in boot means long cable runs and still pulling the interior out.
IIRC the issue has been covered in the 166 section but chances are it now sits in the ICE lounge area, so should be stuff on there. Overall the DSP is probably easier as is just a matter of connecting it up, no decision making involved, its all tested and works, but even so the standard speakers are not the best once fitted which you may wish to change as well.