would bump up the contrast. I like that its not the overcooked type HDR, but its just not an interesting composition so there is only so much you can do with trying to make it better with HDR. what program are you using for the HDR?
never tried Nik HDR. Photomatix will look completely different though, it has a tendency to go pretty far on dynamic range. Ever try Enfuse plugin (if you have lightroom).
you just don't seem to be getting a lot of additional dynamic range out of the Nik HDR.