Mids and highs. The 500s have a pronounced and thick mid all the way to about 8-10khz shy of where you'd start getting an airy treble. This particular 500 however had a grill mod done, I can't say how it differs from a stock 500. The stock he-4 had a more pronounced treble range than mid. I wouldn't say the mids were recessed but it is not as thick as the 500s and the pronounced treble can make it seem to take a step back on the stage. The presentation is definitely front row listening on the stock he4 but instrument separation is very good, albeit when the treble tracks get cooking in big orchestras or soundtracks the mids and treble can sound cramped.
Now the he4 with 3 mods takes on a different sound. With the star felt removed from the driver, pleather pads and grill off (for now until I make some larger opening grills), the inst separation gets even better. The front row listening now is more like 2nd or 3rd row, similar to the grill replaced he500. The treble is less pronounced but still very well presented, the mids and lows stay the same but now the mids come out a bit more. What you get is a flat response with a polite treble, great for the treble sensitive like myself. This is more preference, the treble isn't too hot to begin with just a little much for me (unless you're dealing with poorly mastered tracks then the he4 can get treble hot in a hurry).
The bass between the two is about the same as far as how deep it goes and clarity but the he4 has a slight one up on slam and quantity.
For the price, the he4 has excellent value. If you have the power to run them already, I would say it had better performance to price ratio than the 500s. (I'd think Darth Vader would have liked them too since he has the power for em but they're a little bright for his style)
What's interesting is the new hf line being single ended with the same light weight as the he4, yet seemingly only to be as efficient if not more efficient than the he400. Can't wait to audition both the new hps, more so on the 560 and sometime in the near future. Very excited for the new lineup.
Comparisons were done on a dedicated Sabre AudioGD DAC to a lisa 3 amp for both he4 and he500. I own the he4 so I have longer experience with it's sound than the 500s.