Pad mods.
I have spent now 6+ months with my 507, and it can be considered broken in. The claimed brightness never bothered me (perhaps due to the amp pairing, and also to my aging ears), but indeed sounded a little bit lighter than I wished. Instruments do have that nice body resonance, but slightly on the ethereal side of reality.
I started playing with reversible mods, mostly with the earpads. The leather pads of the 507 fold over the piece which is glued to the body, so that I could safely stuff in small blobs of natural wool filling for experimenting. The wool blobs were of 5-8 mm diameter, available as replacement filling for sleeping pillows with natural wool fillings.
First I lightly stuffed the four corners, then very lightly the sides too. The sound became darker, with more bass and less treble, more distant with less details. It was too much for me. Then I took out the framed cloth which covers the drivers. Sounded cleaner, with virtually no treble etch (but without the wool filling, it does sound etched without it). However, I wanted a more open sound.
I took out all stuffing and started to experiment corner by corner. The lower corner behind the ears is worth stuffing fully. It improved everything, more bass, calmer treble, better imaging, and better comfort. The upper back corner behind the ears wants to take less stuffing - too much of it causes too dark sound. For me, the front corners (bottom and up) are the most sensitive to stuffing, I could hear the difference that even a single blob made. I have just put one blob or two in the lower front corner and left the upper front one empty.
It's surprisingly good now. I used classical, instrumental and electronic music for testing, now all of them sound better. The bass has got a nice touch of slam and attack, and the treble sounds "calmer". It does not sound dark or shut in, it's still very open sounding, but not any more too light/ethereal. The good news is the balance can be changed very easily by changing the amount of stuffing, so everyone can dial in the desired effect. If I will have the opportunity, I will compare it with other headphones again, but this change cured by upgraditis for a while. I really enjoy the 507. When I bought it, I preferred it over the Senn HD800, Hifiman HE500, HE6, Audeze LCD-2r2 and even the LCD3. Now it is even better, especially for the price.
. I tried putting back and forth the cover several times, but after listening over 2-3 CD's I removed it again, and there is no fear which would make me putting that one back. There is a metallic mesh protecting the driver, which of course smears the sound through secondary reflections, but still sounds more clear with much finer gradations of detail, than through the cloth cover provided by Stax. On the back side, they use a very loose texture foam, similar to the active carbon filters. I may try other materials for dust cover, but for now it is very good without it 
