I took the foam backing out of the M565. The driver was much harder to get out of the cup than I thought it would be. After taking out the 4 screws the driver would not come free and there wasn't enough of a gap around it to get anything in there to pry it up. I ended up having to push the driver out from the back. I stuck a dental pick through the grill and pushed it out. I took extra care to make sure I was pushing on plastic housing and not sticking the pick anywhere close to the actual planar driver (the last thing I wanted was to poke a hole in my driver!). That was the hardest part of the mod, by far.
The sound seems to be improved. It still sounds dark but less congested with more detail but it's still laid back with a smooth sound. It's not sibilant and didn't create any unwanted spikes in the frequency that I could tell. It's definitely a keeper and for $80 it was a killer deal. No regrets. It is marginally uglier after removing the foam though. Seeing the exposed driver wires isn't particularly attractive. Maybe I will put in some acoustically-transparent speaker cloth in there some day to hide those wires.
I have been going back and forth between my HE4XX and the M565 with the first 5 or so songs on that playlist. Imaging and soundstage seem very similar between the two headphones. The HE4XX are harder to drive, I have turn up my amp more for the 4XX (8 o'clock for the M565 vs. 10 or 11 o'clock for the 4XX). All listening was done with a Emotiva BasX A-100 amp with a SMSL Sanskrit 6th Edition DAC. No EQ was used. I don't have Spotify premium so I think the bitrate is of the music is like 256 kbps (don't quote me on that). Sorry if this comparison is a little scatter-brained; as I listened I just jotted down what I was hearing or what I noticed.
Breezeblocks:
HE4XX:
Highly detailed, but at the expense of some instrument separation because during the chorus there is a quite a bit going on. However, there is some nice texture to the vocals and the treble of the percussion. This song feels more dynamic on the 4XX.
M565:
More laid back vocal mix than the 4XX. As a result, when the backup vocals hit at the 11 second mark it's much less dynamic on the M565. Whereas the on 4XX it almost surprised me when they hit.
Super Bon Bon:
HE4XX:
Bass is more laid back compared to the M565, allowing more texture across the spectrum. The vocals take front and center instead of fighting with the bass for prominence. The background clicks (maybe from a vinyl record?) are more distracting though.
M565:
The bass in the song punches much harder on the M565 but at the expense of the vocal clarity. The clicks in the background take much more of a backseat to the vocals and bass.
Walk On the Wild Side:
HE4XX:
Again, the vocals are more prominent with the 4XX, probably because there is less mid bass to distract from the lower male vocals. The additional treble makes this song also feel a little more engaging too.
M565:
The guitar isn't as detailed and has less texture on the M565. Vocals are smoother, but get a little more lost thanks to the boosted mid bass.
Good Morning Little School Girl:
HE4XX:
The treble is nice and crisp adding detail but as a result the sound feels less spacious. Bass is right were it should be for this song. Giving just enough kick where needed without distracting from the vocals or other instruments.
M565:
Sound feels pretty spacious. Everything is nice and smooth. The bass can get a little carried away sometimes, at 1:41 for instance.
Rill Rill:
HE4XX:
This one is much harder to listen to on the 4XX. It reveals much more of the imperfections in the music. The "ess" sounds in particular are rather sibilant and sounds like they didn't use a de-ess filter at all when they mastered it. As expected, the bass hit less hard but retains the texture.
M565:
It's much easier on the ears with the M565. The treble details (or in this song's case, imperfections) are much less noticeable and the bass thumps harder.
Conclusion:
Both are good but have different strengths. The M565 is smoother and more laid back at the expense of some detail but has better separation. The HE4XX are more detailed in the mids and treble (sometimes at the expense of the song) with better controlled bass that affects the mids less often. The M565 is like driving a Cadillac down the highway, smooth, comfortable, and laid back driving but a more muted driving experience. The HE4XX on the other hand is like driving a hot hatch. More peppy and engaging but if the road is rough your butt is going to be sore at the end of the drive because you will feel every bump. If I had to sum up their signatures with a single word it would be "Smooth" for the M565 and "Crisp" for the HE4XX . I hope that helps. If you have any questions or need more clarification, don't hesitate to ask.