Cartridge loading

I have a Benz Micro Gullwing low output cartridge and supposedly has a load rating of >400 ohms. I am using an Einstein Turntable's Choice phono section which had 69db of gain and has four sets of interchangeable plugs to vary the load. The problem is, the highest load available is only 300.

When I checked with Einstein they said there is a 500 ohm "load" when using NO plug. They don't make any custom plugs so I am stuck with one of the four sets of plugs that come with the preamp. What does that mean sonically or electrically? What changes could one expect if a 400 or even 1000 ohm load could be applied? Is there an inherent problem with using no plug in the SE slot?

Thanks in advance.