The 500 has a little more bass than the 400, and the HS50 has better bass response than either RIG model. As I mentioned in my previous reply, I’d put them all in the same tier of quality sound-wise and I think they all sound “good.” Without knowing your personal tastes I can’t tell you which one you’d think sounds best.
I recommend checking out some graph comparisons over at Rtings.com if you want to get a better idea of the relative differences between different units. My reviews more or less standalone unless they’re explicit comparison pieces, and represent a snapshot of how I felt at the time.
I don’t keep a vast library of headsets on hand. I only have around six at a time and I sell the rest to help me afford to do this.
All of the models you’re looking at are easy recommendations, for me the 400 and the HS50 are easier to recommend than the current 500 lineup. It just kind of depends now on what your personal tastes are.