J$ beyers are really dynamic with HE400 (possibly the most dynamic out of all earpads I've tried, but the jergpad is quite a close second), but the tonality just doesn't seem to synergize well with HE400.
Too much of HE400's treble is brought out with the J$'s, the midrange is very clean-sounding but again, the tonality doesn't synergize. Overall the resulting sound is very detailed, dynamic, with terrific bass, but bright and U-shaped.
IMO the jergpads fix the HE400's upper midrange dip and treble peak by a lot. The resulting sound is still HE400 (laid-back and sparkle treble) but not nearly as exaggerated as with other pads. That is why I kept stressing that vocals and rock actually sound nice on HE400 now, because the perceived tonal balance became a lot closer to neutral in the upper end of the frequency spectrum with the jergpads, so voices and string instruments are brought out and bloom much more.
In terms of comfort, jergpad is identical to stock pleathers sans the dust screen; flat, low clamp, and relatively pliable memory foam. J$ is softer but thicker - so if you don't mind the clamp it would be more comfortable, otherwise it might be less comfortable.
I will try down the line but I'm quite familiar with how the pleather dust screen modifies the sound so I can speculate with some confidence.
The pleather dust screen would do 1 main thing: subdue the whole of upper midrange ~ treble and roll off the extremes of treble. Now that might be ideal sonically to some, but I feel that the jergpad without screen is already sufficiently balanced-sounding with regards to mids vs treble levels, and further lowering of the treble might be detrimental.
One recommendation I would heartily recommend is the dust screens from velour pads. Those are I'd say close to 100% acoustically transparent, so the resulting sound would be practically identical to the sound of the screen-less jergpads. However that means you have to sacrifice a pair of velours ($10+shipping).
I think everyone should at least try the screen-less jergpads first, and then decide what to do with the dust screen situation. Really all you need is some sort of stretchy fairly transparent nylon fabric, plus some superglue, and just glue dat thang onto the mounting ring's flat side.