My hamstrings and quads are really strong, but because I have built them up so much, the backs of my knees always look bent. I have completely open hips, but the backs of my legs are so tight that bending over them in parallel and putting my palms on the floor is painful. My hamstrings especially are so tight and compact that they prevent the back of my leg from lengthening out and looking completely straight or (what I am aiming for) slightly hyperextended. This is prominently evident when I do a developpe or grand battement to the front, because I will get my leg up there but the tightness of my hamstring will cause the leg to bend slightly - even if I am straightening it and pulling up my knee cap as much as possible. I could be slightly HYPOextended, but one of our company members told me that it is most likely due to having tight quads and hamstrings. I am looking for some stretches that will lengthen my upper leg muscles and ultimately the backs of my knees - all tips, tricks, and advice appreciated!

Seeing as quads and hamstrings are opposing muscles it is very unlikely that they are both tight.

If you have tight hamstrings the quads will be loser and visa versa, that's sort of how muscles work.