HE500s with warm-sounding pads (stock or modded pleather earpads being the most obvious) have very liquid upper end, by liquid I mean very smooth yet not overly recessed. It's a far cry from HE400's upper midrange ~ treble behaviour. I can crank the volume up all the way through HE500, and the treble is the last thing to irritate my hearing.
If you use velour earpads or very deep non-Hifiman earpads on HE500s, on the other hand, you might find the upper midrange a bit too aggressive for nonfatiguing long-period listening, so be aware.
LCD2, especially the current version (revision 2), is pretty forward from what I've auditioned. About as forward as HE500+velours, more forward than HE500+pleathers, and a lot more forward than HE400. LCD2s do have better depth perception (projection of sound in the forward direction) than any of the HE400/500, but IMO its width perception is a bit lacking.
HE500's bass has much better extension than HE400, and it's a bit fatter-sounding (less clean, more rumbly) too. It's also a tad less prominent than HE400's bass, relative to midrange/treble.
tl;dr depending on your priorities, either HE500 or LCD2 would be appreciable upgrades for you.
Jerg... I see you so often being helpful. Thanks bro. Keep keeping it simple and informatively to the point.
Edited by grizzlybeast - 9/7/13 at 1:13pm