Increasing the earthy flavor of beets?
Variety-wise there are many beet types, I have grown a few and I've found that red beet varieties have earthier flavors rather than golden ones. Larger beets of the same variety tend to be less sweet and more earthy than smaller ones.  I'm not going to weigh in anymore on varieties as I simply don't know, the gardening stackexchange site might be a better place for that question. 

Cooking-wise if you want an earthy flavor then I'd suggest baking the beets as opposed to steaming them (don't boil them, ever) as the dry oven will help concentrate their flavor. Leave the skin on, then peel them once the cooking process is done. It isn't going to make a massive difference, but it should be noticeable depending on the variety.