Dude, I read your HE-500/HE-4 comparison...I think about the HE-500 all the time and I have you to blame for!
About the bass of the HE-4...it is very, hmm, "good." (<-- I'm so descriptive and accurate aren't I?)
It extends very low, is of ample quantity, and got some punch to it. I like punchiness.
I wouldn't mind having more, but that's because my previous cans were Ultrasones. :P
Then again, if the given the option I don't think I would actually want more bass. The amount it has right now is probably what makes them sound so good with all genres.
I guess I would call this the "ideal" bass. I had the K702s for a short while before the HE-4s. They are no longer with me. ;)
I'll agree with you about the extension. It's bass extension is literally the reason I got rid of my HD598. It was not a fair fight at all...
HE-500 will give you that little bit "more". It's got a better texture and richness to it as well. Possibly the best I've heard in a headphone.
And I'll agree again about the "ideal" bass. Less is definitely more. But the HE-500 is definitely not too much by any standards. It's perfection.