I think I might give some of these mods a try as I think the HE-500 are one of the best values you can get right now. They are damn good stock so lets give them a different take.
I've papered this place with HE-500 mod info (based on those before me), and lots of experimentation. Here is a short list w/ prices paid in order of efficacy (IMO):
1. fuzzor mod; $5, 2 hours including 'tearing' them down and putting them together. Lots of guidance w/ pics right here on Head-Fi (circa 2011-14). Quiets amusical reverb/reflections in mids and esp highs. Couldn't frankly stand them without this mod.
2. rear screen; $0 or < $10. Remove, retain ring; If you are not worried about the drivers getting poked, leave open. If you are, get 1/2" ground cloth, cut to size and get the 4 closest cut metal ends into the nearest retainer ring hole, you probably have to trim again, and "dial" in the fit, success when flat and the ring fits in normally. You will lose some bass, but under 400 Hz note how much in the way of added reverb/cruft gets removed. Note: If you cook, or even have a natural or propane heater in the room with your cans, I'd put them in air proof container/bag except when using. That also keeps the dust away, which you somewhat lost by removing the screens.
3. pads: only a few seem to be approved by the cognoscenti. I have not heard the 500's at their best with a non angled pad - or with a velour or fenestrated pads. My search with a big pointer from the 500 and HE-6 mod pages is the MrSpeakers Ether Flow Angled. $85. Really tightens individual instrument focus, image depth, and the flatest measured treble of any pad. Bass strong but vents enough not to be fat. With screens on, soundstage width with these not the best, but with screens off or 1/2" groundcloth. Wow.
4. While you have the 500 opened up during #1, add dynamat, which tightens the bass. I like using it with the metal backing off. Roll up some, and squeeze into the edge between the inside of the outer can, and the driver frame. all the way around, don't let any random bits float around to touch the mylar (not sure if that would hurt it, but why temp fate?). On the inside the smallest vertical holes (one on each outside edge), don't put down felt, use dynamat (blocked by every pad I used, so felt does you no good in those slots). The 1/2" screen is pretty "live" once you are sure you won't be opening up for some time, put dynamat down around the edge. $20.
5. The stock cable sucks period. Microphonic, over emphasis in the treble. Copper, good OFC, nice connectors, avoid crazy expensive stuff. I got a Black Dragon v1 for $40 used, works great. Helps quiet down the cymbals and other glassy high end sounds, seems to focus the mids better, more slam in the bass.
6. Headband. Not aural per se but if it helps you keep them on longer maybe it is. I subbed in the band from my 4XX, and after that never once thought it was too heavy. Lots of ways to this goal. Cost: ?
So, $360 for the can, and $160 mods (not counting headband), you get a can that SilverEars thinks is nearly as good as his unmodified HE-6. I won't go that far, the 6 has a bass wallop that no configuration of my 500 can match. The HE-6 also has treble detail that my 500 does not. But my 500 has a bigger soundstage, better mids, and less annoying in the treble at times. I have no doubt that a well modded HE-6 would smash my 500. But I also think if HFM took as much time with the surrounding bits as with the driver of the HE-6, and issued it today, that it would easily compete in the +$2000 market.
