Thank you once again for your help, Stillhart.
It sounds like you really enjoy the HP100's. If I were to change my mind and go for a closed headphone, is the HP100 the best choice at the sub-$300 price range for immersive (not competitive) PC gaming? I've heard the DT770's are pretty good too, though perhaps the 80ohm version would need a better amp than the U3.
I almost got the DT770 before I got the HP100. I was told by a couple people that I wouldn't like the DT770, given my preference for the Q701's sound signature. I'm sure they're great, they just sound different; once you have a better idea of the sound signature you like, it'll make decisions like this easier. Unfortunately, that will only come with experience.
Score! I'm sure you'll be happy with them for the price. And if not, should be easy enough to sell them for the same price you paid (or more!).