I have heard the 501s, and I don't think I agree with you that they lack bass completely. They do sound kind of lean, and the bass that's there is not overly prominent - but it is there. I much prefer it to the bass I heard on Senn 580s and 590s - those i felt had too much bass that was a bit woolly or flabby.
Yesterday I compared 401s with my old Audio-Technica ATH-70s - the old ones are quite detailed, but in comparison to the 401s have hardly any bass. Yet when I listen to them at home, I rarely find myself wishing for more bass. At a reasonable volume level, the bass is nicely there.
I guess I'm just saying that I think the 501s have ample bass, and that statement is based on my feeling that one gets used to a certain sound and enjoys it, particularly when you're not comparing it A/B with something else. And of course there's personal preference - I seem to prefer a leaner sound.