Actually the botom rack is at the same level regardless of if you have the 3rd rack option or not.
If you did a side by side comparison of all the deep tub dishwashers out there today, you will see they are all right about the same height for the lower rack.

Also the 500 series is not Bosch's quietest dishwashers. They are very quiet but not as quiet as the 800 or the 800+ d/w's

Interesting that they are all that low now. I thought it might also have to do with being able to raise and lower the second rack.

Yes, I know the 500 is not their quietest; I simply mentioned it to hopefully benefit others. I read many reviews which indicated the 500 series was silent.