I see. I have been listening to K7XX most the last few days; the H10 is so good for them that it actually is now surpassing my Lyr, which I prevously thought was a match made in heaven for these AKGs.
Here is something I will say that may or may not be relevant to you.
Someone on the K7XX thread criticize the headphone for revealing a hole in its center image.
When I first plugged in the K7XXs, I was immediately unimpressed, like within a minute, compared to my Lyr, which made them so graceful.
But these last few days, I have been listening to the K7XX out of the H10 exclusively, and these experiences have constituted some of the most memorable I have had since joining the hobby.
To indirectly respond to your query while yoking together the tedious narrative above: after I saw you post, I changed my gain to 0 and noticed a hole in the soundstage, which, remember, others have blamed the K7XXs for. This hole reminded me of the first time I dismissed the H10 as being responsible.
Switching to +6 gain, I noticed this hole completely disappeared.
Somehow, upping the gain filled in the K7XX. I would suspect, then, that higher gain would inflect the reception of your HE-500. The question is really which setting most appropriately suits your tastes.