You are certainly a lucky guy with a WG Karree tourbillon. I came across a platinum one owned by a friend a few years back. As I understand, it was a flying tourbillon and the rate the tourbillon spins around was incredible. Couldn't even see clearly what was going on in the cage. Much more mesmerising than the Breguet design. Beautiful.
I have experience with these auction houses (Sotheby's etc) and they seem to prefer setting the price/value/low estimate to be very low in order to attract bidder's attention. They believe in setting an atmosphere in the bidding room. I think some recent limited round case GO tourbillons that we still see in AD's were sold in antiquorum for near retail and I cannot believe a discontinued limited one like yours would go at 1/3 the price you bought it for unless you were paying over retail initially. You will be pleasantly surprised how much it will fetch in auction.
With PP, I think because there is a big market with well known auction prices, as you can see different ref. up for auction all the time, so the auction houses cannot set the estimates too low. Compare a 1982 Mouton magnum to a 1955 DRC Richebourg magnum.