Welcome to the "you underpowered them" corner. When I first started listening to the LCD and HE, both sounded like you described. Cavernous and diffuse centerstage. I could get plenty loud on my headphone amps and even the couple speaker amps I used and the left and right staging was beautiful. It wasn't until I put some serious speaker amp power to the HE that the staging and echo effect stopped and the intimate feel and cohesive soundstage was offered up. Lots of people think volume is the measure of an amp. This journey has shown me an amp has a lot of influence on the sound. The most influence is the spatial cues that form the soundstage. If you think the 500s are strange, listen to the 6s underpowered.
I like your writing style, Happy Camper.
I'm not sure I understand the difference between speaker amps and "serious speaker amp power". I used 80 watts per channel, through the headphone jack, of course. Are you talking from the speaker outs?
You know, no reflection on you or anyone else here, but I get kind of bored with this "if you'd used the right amp/more powerful amp all those faults would have disappeared" thing. It's not that I doubt it; it's just that I feel plugging a pair of headphones into an 80 watts per channel amp ought to be enough, and if it isn't then I'm happy to find a headphone that's easier to please. I had the LCD-2 Mk1 and never heard any cavernous effects, so why should the HE-500 be so picky just because it's an ortho? Why do you have to wrestle these headphones to the ground and bitch slap them before they sound any good? Nah, I'm too old and grumpy to bother. I'll just stick with my inferior, mid-fi DT880, revel in its peerless comfort and try to pretend that it's actually a $2000 Superphone.