Re: Dm as the saddest key
I really doubt specific Hz frequencies have much to do with anything, the average person's ability to remember pitches is pretty hopeless (intervals another matter). Even Mozart was only sensitive to something like a couple of cents (and that's sensing when two different pitches are out of tune by a few cents). Consider that our sense of pitch is sensed by a series of tiny hairs placed on a spiral of increasing curvature...I think you will agree that context has much more to do than absolute positions.
But yeah, open strings are more powerful, richer, and the minorey thing about the minor chord is the b3, so if that's on one or two high open strings, it'll tend to be pretty big.