Wanted to follow up real quick that I took multiple measurements of that channel, and though I didn't necessarily remove all front damping to verify that was affecting measurement, it would appear distortion is reduced in the midrange with this config.

Stock PM-2 sounds OK, just wasn't stellar in my mind. Overall sounded wimpier and less balanced or generally less capable than the HD600 or HD650. Its main selling point was its ability to run from about anything pretty well along with looks, build, and fit or comfort. I think different pads alone can make a positive difference, as the stock PM-2 pads leave something to be desired with the sound. My goal with this mod was to see if I could tune them more to my liking with the most basic and cheap modding/damping materials already in my hands. Nothing fancy, nothing advanced, and nothing that took a whole lot of time to implement beyond going through quite a few experiments.

After a few different experiments, I decided I wanted to target a sound that was a bit more bass emphasized but otherwise fairly balanced beyond that. Partly because I already have the HD650 if I want a relatively neutral sound and wanted something that was more fun sounding, and partly because I think the OPPO driver can be a bit weak sounding down low even if it measures about the same as the Senns, for example. I wanted to give it a bit of an extra kick and sense of thickness without killing the mids or treble.

For internal damping, I've attached a picture of the configuration I've stopped at a while ago. See below. Sorry about the crappy paint job. It's just not worth opening them up just for a shot of the internals when I can do it in paint in seconds. Basically, you have the eight middle squares (or, rather, two middle rows) blocked and sealed off, the four "edges" in each "corner" (above and below) are half covered. Then the two upper and lower squares in the middle area are half sealed. These are all still covered with drawer liner.

The damping foam I use is still the same 3-layered configuration. The squares closed with foam more or less control the bass quantity, so it's fairly tunable in that regard. I did try closing the four middle columns before, and while that actually measured fairly flat in the bass, it wasn't what I was going for and may have made things sound too thin. There are other tradeoffs too when closing these squares. More open squares will generally give a more open sound at the expense of it sounding more diffuse, less focused, and, of course, bassier. Closing too many, or closing squares in a certain config, can start to make things sound too thin and sharp, IME. The config I settled on gives me that extra bass I wanted and balances the sense of openness with focus.

Alternatively, you can use more or less dense damping material for the same purposes. The stuff I use is considerably less dense overall than the stock OPPO damping material, so I have to rely more on controlling airflow/venting from an enclosure perspective for bass tuning. Personally, I think the stock OPPO stuff is too thick or dense and stifles the driver's performance to a degree.

As for front damping, when compared to my earlier experiments with stock internal damping (shortly after I got the PM-2), I'm now just using discs of that 1/8" open-cell foam to help further smooth things out. This really comes down to personal preference. It will sound brighter and sharper with nothing in front of the driver. Smoother and more laid-back the more stuff you put in front of it. I will likely settle on having one donut of this foam and one full disc in front of the driver for my tastes, but others may like less material (or, I suppose, even thicker material).

I will say that I most prefer the velour pads with this config...and just in general with the OPPO PM-1/2. Stock PM-2 pads measure pretty funky with my mods.

There does seem to be a spot around 400Hz, maybe a resonance spot or something where it shows a small dip in measurements, that can become a bit more pronounced with these mods. Depends on how much bass you want. This is audible but not necessarily detrimental overall. A bit of PEQ can show you what I'm talking about. I believe you'd need more advanced mods, perhaps even enclosure, baffle, or driver modifications, to alleviate that. I also noticed my mods seemed to clean up THD results on the left channel. After further testing, this did not appear to be the case on the right channel, for whatever reason.

Again, these are super basic, super cheap, super easy mods. I think they work well for my needs and my goals - namely the PM-2 now sounds more fun and powerful down low and more balanced up top. Just more life to them overall. Bass levels are tunable. I think it's a step up from stock, but don't expect everything to be miraculously solved or amazing about them. If you want something more advanced, there's the upcoming Audio Zenith mod (pricey) at least. Not sure if anyone else has any mods they'll be putting out for these in the future or not. The AZ mod targets a different sound than I was (more linear, less bassy, generally more laid-back and a touch darker sounding), but the more advanced nature of the mods do bring more benefits to the table beyond that.

Hopefully that answers all your questions and more! Let me know if you need anything else or have other questions. Two points of advice if you try something like this mod. First, make sure you put any adhesive material against some fabric or something (peel and re-stick a few times) so that it will be easier to remove later if need be. Second, be careful tightening the baffle screws on the PM-1/2, because you can start to wear down the plastic if you get too forceful with them like I did.

If you're talking about Alex's Audio Zenith PMx2, then no. I've also made further changes to my mod in the past couple months. You know how it is...get too used to things and wrapped in your mods, so you take a break, come back, and find something you think you could improve. Lots of the same ideas still, just slightly different implementations and further tweaks.

I'll post some new measurements and stuff on my mod soon. Maybe. Trying to be more patient, since I always seem to find something to tweak and have to go back on what I brought up earlier.

There's a thread on the PMx2 with measurements and impressions from Ultra and I: http://www.changstar.com/index.php/topic,2117.0.html

Feel free to chime in if you heard these and have thoughts you want to share.