VDH Colibri XPP. Best Load impedance?

Josef, as stated in several other threads, unless your preamp is sensitive to Radio Frequency Interference (RFI) then the loading will not be critical. If you find that it is, then your preamp is having a problem (and if it were me, I would consider replacing the preamp). Emil_f's suggestion is thus not a bad one.

The inductance of the cartridge is what speaks to the loading value. In this case the 500 ohm resistance value is also in question, but even if it were so, the inductance is what interacts with the phono cable capacitance to set up the tuned RF circuit I alluded to earlier.

In short, it appears that Jpetek's preamp section is indeed sensitive to RFI generated by the afore-mentioned RF tuned circuit. So the value he likes is likely going to work only for him as it is dependent on the capacitance of his interconnect cable.