This is one of the most interesting posts I've read - thank you!
The topic of variable layering of least dense materials closest to the driver to most dense materials at the internal cup surface for reducing reflections has been discussed on this forum, but not in as much detail as you described in your post. Dynamat and other materials have been tried on the internal cup surface and covered with less dense materials including open cell acoustic foam, cotton, fiberglass, etc., with consequent reduction of cup volume that may affect the sound. Several people, myself included, have tried Dynamat in the cups with a covering of Paxmate or similar acoustic foam. I did not hear any difference compared to Paxmate, alone. I don't remember if anyone else posted their impressions with this configuration. Armaegis pointed out that Dynamat may add mass loading to the cup if placed on the internal cup surface but would not result in reducing reflections because its constrained layer is not against the cup surface and is essentially mounted backwards from its intended design. (Armaegis: correct me if I am mistaken about what you said.) This got me to thinking about how to better (?) make use of Dynamat. On BMF 5 and 6 mod configurations, I removed the dust cover and its adhesive residue from the ear side of the baffle surrounding the driver. Next, I placed Dynamat on the exposed plastic surface surrounding the drivers. To diminish reflections from the metallic-looking Dynamat constrained layer, I covered it with self-adhesive felt.
Armaegis, and I think leeperry (?), commented that I over-damped the backs of the baffles by using too much plasticine and suggested using tungsten putty or lead shot. BTW, to my ears there are advantages to this "super mass loading" about which I've already commented that may be offset by excessive weight. I researched tungsten putty and found it available for $10 to $15 per oz from various venders - mostly for pinewood derby cars and fishing. I found fishing tungsten putty for about the same price and decided to formulate my own. I applied my formula to the back of the baffles in various quantities and placements and posted the results. A week or so later, I wanted to compare my homemade tungsten putty to a commercial product. I ordered Kryston Heavy Metal Extra and quickly concluded that it is too hard, too dense, and too expensive to be feasible for our purposes - see post 4513 for comparative weights of plasticine, my tungsten putty (current formula), Kryston tungsten putty, stock T50RP, "Less Is More" mod, and "Transpore 1.5" mod.
Several weeks ago mrspeakers posted a mod that includes filling the cup outer rings with plasticine to produce a better seal. I don't remember who posted a mod including Paxmate in the cup outer rings (sorry). I have one mod with plasticine and another with Paxmate in the cup outer rings. I don't know if these mods make a sonic difference, or not, but think they can't hurt as long as they don't stress the cups' plastic threads. To this end, I always apply pressure to compress the baffle and cup when removing the 4 baffle screws and avoid over-tightening the screws when putting the cups/baffles back together.
AnotherNoob provided a link a couple of weeks ago for a manufacturer of another open cell acoustic foam product - AcoustiPack Lite - which has similar foam as Paxmate Plus mounted to a constrained layer. I bought some and tried it with similar, but slightly different, results compared to Paxmate. To my ears, Paxmate offers a warmer sound to AcoustiPack's more analytic sound. Further visual and tactile comparisons show that AcoustiPack Lite is heavier than Paxmate Plus but it's open cell foam is only about 50% that of Paxmate; both are 4 mm thick.
After reading your post, I am now going to place AcoustiPack Lite in the cup bottoms (not the sides) to take advantage of its constrained layer (most dense layer the greatest distance from the back of the drivers). Next, on top of the AcoustiPack I am going to add 1 layer of Paxmate (not the sides) to take advantage of its (I presume) less dense and greater quantity of open cell foam in closer proximity to the back of the drivers. Everything else in my "Less Is More" and "Transpore 1.5" mods will remain the same, as described in post 4513 a few pages back, including 1/2 of one medium cotton ball loosely teased and placed between the Paxmate and the back of the driver - the least dense material closest to the back of the driver.
A few months ago I noticed that only 1 of the 4 baffle holes surrounding the back of the drivers is actually open all the way through to the other side of the baffle. I assumed this was designed as a means of equalizing the pressure between the cup chamber and the ear pad-to-head chamber. I referred to this single open hole as the pressure equalization vent - but that's just my layman's wilda*s guess. I remember probing the other 3 holes around the driver using a paperclip and was stopped by what I assumed at the time was the baffle dust cover. I thought that they were purposely designed with the dust cover over them as a means of, as you just described, controlling the resistance or back pressure. Two days ago, I was looking at these 3 holes and thought this is another potential mod to explore. So, I decided to open them by using a needle to make a pilot hole and then push a paperclip all the way through the dust cover to widen the baffle opening to the same size as the driver holes. To my surprise, this was not possible because they are not open at all; I hit the plastic baffle instead of the expected dust cover. So, I got out my drill and used a 1/16" bit that is just large enough to fit the inside diameter of the driver "vent" holes. I drilled them out making them actual open vents so that there are now 4 open baffle vent holes instead of just one. I have been playing around with them fully open, fully closed with tape, and partially open covered with thin felt. I have not yet tested all permutations of # of holes X fully open X fully closed X partially closed with a layer of thin felt over them on the ear side of the baffles. I did, however, notice right off a change in the the SQ and what seemed like more bass when all 4 are fully open.
BTW, would you describe "bass rings?"
Keep the ideas coming.
What would that preparation be? Mass loading cups with dynamat, mass loading baffles with plasticine/putty AND also loading baffles on the earside as well. Covering all that with paxmate - both the cup and baffle from both sides. Also covering inner side of earpads so there would be even less reflection.
Note: in optics, there is reflection when light passes boundary between substances with different refractive indices. The less the difference, the less reflection. It applies to sound as well. To minimise reflection, one should use a several-layer system to deflect and absorb the sound waves. Something very diffuse and light near driver, then something denser, and so on and so on. The organic dried sponges seem to be a good choice for the near-driver layer. Then, teased cotton balls. Then, paxmate, then, dynamat and then the cups. There also may be a denser layer of cotton between paxmate and dynamat, depends on the density. Making cotton ball stuffing with different density across its thickness is also a good idea - more teased near driver, denser near paxmate.
The second stage of preparation would be elimination of uncontrolled variables. RP2 do some of that elimination by lining rim of the baffle with plasticine. Other uncontrolled variables are leaks from under earpads and leaks between head and earpad. First kind of leak is not hard to address - stock-like glued-on pads should be properly glued on, and with flapped pads, one might put something viscous under the flaps, but I don't think it is necessary. On the second kind of leak, leather earpads have little leak, and gel earpads have even less leak - the more isolation, the less leak. Sub-bass requires proper seal.
Now, there comes tuning. I see three things to tune: driver damping on front and rear, ports on cup, and ports on baffle. Again, tuning should first result in "proper", fast and detailed sound, and then it should be tuned further to get proper tonal balance. Bad thing, without measurements, we can't be sure what really each measure does. Seems that closing cup ports results in damping resonances and perhaps shifting them lower. However, it comes at expense of compressing bass frequencies, as air has nowhere to move.
Baffle ports seem to be overlooked. They also play role in bass, and subbass, as they leak air between front and rear. Covering them overdamps the driver and reduces bass, while having them open results in roll-off at subbass, albeit having more of the other bass frequencies. I think, they should be controlled - not by opening or closing them completely, but by changing their resistance - covering them, for example, with felt which passes air through but restricts the flow somewhat.
Now, the driver damping itself.. it is the most questionable area, and I don't have expertise with it. I only have some thoughts on reflex dots and bass rings. Reflex dots shouldn't really reflect a lot of sound when placed right on the driver, but they surely add damping that way - they are solid and don't pass air. So high-excursion bass waves get damped, and they-re mainly bass, so there's less bass and hence, more treble - as everything is relative. Still, I don't think anythink rigid and reflective is good. So instead of reflex dot, I'd use a damper dot, if there would be any need to boost treble.
Bass rings, I think, should be the last thing to implement, exactly to tune the tonal balance. I would implement one on the front, to further reduce phase shift in highs - it may reduce percieved amount of highs, but make responce more extended due to less parasitic sounds. And of course it should indeed boost the bass (relatively).
Thanks you you read it all. Maybe my thoughts would be at least somewhat useful for all of you.