Oddly, as much as the comments above seem quite varied, I think they are all right. Loading recommendations are just that -- rough guides -- and following them closely does not necessarily result in the best sound. It is possible that something WAY off from the recommendation will be best in a particular set up; to the extent your gear allows for some experimentation, do so, because it cannot do any kind of harm to the gear.
I agree that the particular loading that sounds the best is greatly dependent on the interaction between the particular cartridge and phonostage.
From my own experience, once you get beyond something like 150 ohms, higher values (meaning less loading) have somewhat lesser effect on the sound. For example, the difference between 150 and 300 ohms is FAR less significant than the difference between 50 and 100 ohms. A recommendation of greater than 400 ohms means that the manufacturer is suggesting that very little loading is needed with the cartridge. With most phonostages, the difference between 400 ohms, 1,000 ohms and 47,000 ohms would be pretty slight.
I would NOT worry at all about the difference between the greater than 400 and the 500 ohms of the Einstein. Try the 1,000 ohm as well; I would bet that there would be little difference. If, at 500 ohms, the sound seems a little too sibilant, bright, or lacking in bass, then you may have to experiment with increased loading (a lower value, like 100 ohms). If that is the case, it would be helpful to consult with a dealer or repair shop unless you are willing to learn how to create your own loading plugs or solder in resistors; both are fairly easy to do, but, one would need guidance on the matter.