The Diamond 9.0 is rated -6dB at 60Hz (as opposed to -3dB which many manufacturers use), which I think is optimistic (I owned a pair at one time).
Given you're requirements (size and price), I think it's going to be tough to find something that can legitimately playing down to 60Hz.
Is the no rear port restriction b/c they will be placed close to a wall? I have a pair of PSB Image B4's I'd sell, but they're rear ported.