VDH Colibri XPP. Best Load impedance?

Why did you start this thread if you already know the answer.

Well I think your response to Atmasphere above is quite rude.
You asked for advice, then proceeded to dismiss it outright.

The answer to your question is - there is no best load.
It may vary depending on your arm/cartridge combination and phono stage.

The suggested loading on the Colibri I listened to were 100R to 47k. These were written down by VDH in his own handwriting in blue ink.

Personally, I was very happy with 47k.

XPP - Colibri plastic body, platinum coils.
Platinum is much more resistive than copper, so the 500 ohms is plausible.
You should probably try and borrow a phono stage that has unlimited loading options eg Klyne 7, that you can try out.
Alternately there are some phono stages that are current sensing and do not require loading at all - eg Goldmund PH3.
You wont find many prepre's with wide loading options as they tend to be old school with loading down for LOMC's.