Try boosting treble.
If you like S680 better this way, it just means that you like S500's V-shaped signature more than S680's more balanced signature.
I find the 680's to have too much bass (despite it being really tame when compared to how they sounded out of the box) and to be lacking mids (despite most people saying they have good mids). This is my Rockbox EQ for the S500's. Which I lined with felt on the inside of the cups, sealed the bass ports and swaped the stock pads for SJ5 pads. The inside of the pads are also stuffed with cotton. I'd say it is pretty neutral.
balance: 0
bass: -5
treble: 0
channels: stereo
stereo_width: 100
crossfeed: custom
crossfeed direct gain: -15
crossfeed cross gain: -95
crossfeed hf attenuation: -115
crossfeed hf cutoff: 1100
eq enabled: on
eq precut: 0
eq low shelf filter: 40, 28, -7
eq peak filter 1: 130, 16, -7
eq peak filter 2: 290, 61, -6
eq peak filter 3: 2510, 64, -30
eq peak filter 4: 4990, 64, -40
eq peak filter 5: 5160, 64, -22
eq peak filter 6: 6230, 64, -40
eq peak filter 7: 9630, 64, -70
eq peak filter 8: 10430, 64, -80
eq high shelf filter: 11430, 62, -16
dithering enabled: off