pr0n time:
So the headphone consists of a primary, inner chamber and the rest of the cup is the secondary, or bass, chamber. Two small holes in the primary chamber with filters are all that connects the two, very much like closed-back Sennheiser cans.
I later added a 1cm x 1.5cm square of Akasa Paxmate Plus foam on the back of the primary chamber, resulting in clearer, cleaner sound. All that reverb in the secondary chamber masks the bass and the lower midrange.
I also tried out some Dynamat Xtreme on the back and sides of the primary chamber (not inside of it!) and that cleaned up the overall sound and clearered up the bass a bit, but what had the biggest impact was the acoustic foam.
Alternatively, one could try any open-cell acoustic foam with some double-sided adhesive in place of the Akasa for the same effect, just cut it 4mm thick and no more. Additional foam resulted in some small loss of bass quantity and impact, so one 1x1.5x0.4cm square of foam is enough.
Those earpads are really not designed to come off with any ease, they are held in place by those five, round dual-tabs seen around the baffle rear here. I had to open up the cups by cutting a small hole in the dust screen near each of the three screws that secure each baffle and driver to each cup. The white strip seen in the photo is a filtered bass port consisting of three holes. Making the holes bigger from the front side will probably result in increased bass, for anyone interested.
The three smaller screws still in place must be for the baffle face, they are too close to the drivers to have anything to do with the ring that holds the earpad. So accessing the primary chamber's interior and the driver would require removing the earpad first in order to be able to remove the baffle face after removing the screws. Good grief.
The pinout turned out to be ring + sleeve (signal + ground, respectively, and tips are disconnected) after all. I moved the signal wire in each cup to the tip so now I can insert the HM5 cable all the way in and get sound properly. So CHC meant for us to depend on them for cables and earpad replacement. Well, you know what? Screw you, CHC.
Alternatively, a replacement cable would have to be built with regular 3.5mm TRS plugs for the headphone end and have the tips disconnected, just ring + sleeve.
As much as I've improved the sound, there's still room for improvement. I'm thinking that I will have to experiment later with cutting small holes in the inner side of the earpads to give the drivers access to the air in the earpads, resulting in the drivers controlling more air between themselves and your ears, which should maximize clarity, but this can also risk exacerbating treble frequencies to the point that the headphone ends up very shrill and fatiguing if I over do it. Some bass will be traded off as a result, too, but I could probably mitigate that by enlarging the bass port a bit. I'm quite happy with the current sound as it is, a significant improvement over stock. The treble isn't as dark as before, much better now, but clarity decreases with increasing frequency to a small extent, so the lower treble is a little clearer than the mid treble, mid a bit more than high. The sound retains its "V-shaped" fun character as before.