Sadly, all part-exchange valuations feel like a kick where it hurts. I wouldn't get too hung up on it, part-exchange at a place which values it closer to what you believe it's worth, and if you can't find a dealer who can get close, sell it privately or keep hold of it.
The W163 is a great car. I wish we still had ours. I gave ours to my father in law and after a year or so he was concerned that there was some rust showing in the arch and he part-exchanged it for less than I spent on it's last visit to the main dealer, 18 months beforehand!
The trader then used it for 12 months, and didn't even change the registration plates (as my FiL had transferred his off the car). My FiL even received a parking ticket a year later, as the trader was still using it and showing false plates, on holiday in Cornwall! Can you tell I'm still bitter