Hi, I've also got a 500 with a 501 interior and bilge keels. I've never experienced this issue with mine. Most of my drying out is on mud though being on the East Coast and her keels sink all the way in.

I have dried her out on a harder surface on occasion on the town hard and I found that she still sat upright but was very finely balanced in as much as I could rock her quite easily by hand to and fro.  

On my boat the main locker is pretty much chocked full with a dinghy, spare fuel etc so that would add a bit of weight aft, which may make the difference.

Hope this is of some help.

we always dried out pointing uphill given the choice, but we also had an enormous amount of kit in the stern including spare fuel and a large fishermans anchor as a spare.

I was always conscious that she looked front heavy on the hard. When in the yard she always had a prop wedged under the bow.

We are also a late model 500 with a 501 interior and keep Soubrette on a drying mooring.  We have never dried out bow down. We too have a fair bit in the cockpit locker and a water tank under the cockpit floor though that is generally fairly empty. Maybe a key point is the weight of chain in the bow locker - we have 10m on the anchor, 50m of chain might make things very different.  

That said we had an interesting bow down night on our holidays this year when we picked a drying mooring with a hole behind it in Trath Coch on Anglesey,  but the bow still never touched the sand. Makes you rethink your choice of bunk...

Ronan