Three things, the calibration, the contrast, and maybe the sharpness / lens. The hd131xe has been reported to have a mediocre lens. That said, in all but the darker scenes I think you should be able to get just about any projector looking similar in bright scenes to another, but it requires calibrating. As far as the darker scenes, the contrast is what makes the difference.
I haven't heard of people getting 2 bad units in a row very often, but it can happen.
I know you are probably hesitant to try another refurb projector, but JVC has better quality control on their B-Stock units and they come with a 2-year warranty. The JVC's will even beat the Optoma at contrast easily. JVC is an LCOS projector though, so it has a slightly different look than the DLP's.
You can get a JVC B-stock from either AVS Sales (the sponsor of the forums that used to be directly connected to the forums), or you can get them from the Ebay Offical JVC Pro / B-Stock Store.
As far as DLP's better than the hd8200, not anything significantly better that is new for under $5000, but one that should come close is the Sharp xvz-30000, if you can still find them they are around $2,500 sometimes, but they are scarce. The color isn't quite as accurate on the Sharps, but the contrast is about the same as the Optoma hd8200.