So below is the end result of around 6 hours and at least a half dozen mod experiments on my HD 650:
(photo is without grill, but am still using it)
Things tried with bad result:
- Dynamat+butyl on the backside of the grill facing the drivers with a checkered design (4 strips)
Seemed to have only shrank the soundstage
- Covering half, then covering all of the air holes along the back with butyl
Increased the mid-bass hump and overall muddyness
- Trying the exact configuration as @UntilThen used above. On my pair this wasn't enough butyl+dynamat to make any noticeable difference
- Adding small rectangular-ish squares of butyl+dynamat around the 'spider' cone prior to removing it
No noticeable result
- Adding cotton underneath the earpads to further extrude them by around 1/2 deeper than before
Rather than increasing the soundstage, everything sounded more "back row". However the mid-bass hump was completely fixed
Things tried with good result:
- Inserting dynamat+butyl as shown above. I also made the longest strip a bit wider for a possibly better effect
The above config. seems excellent for adding about as much more dynamics as possible without reducing the soundstage. Probably refines clarity a nuance by reducing reflections (see below)
- Removing the 'spider' and foam from the back
Added a bit more clarity
During the process I realised how strange this is to put dynamat in this direction; that is facing
away from the driver, and being in a location that covers nothing besides solid plastic surface. Because generally the purpose of dynamat is to absorb reflections of high frequences which is almost always done
facing the driver. As such, I wasn't surprised that initially I couldn't really hear much difference when it was just the top, middle & bottom strips as UT had used above. When I really started noticing a difference was after I added more, per the photo. Finally this was starting to add a but more punch to the drums. As far as I can tell, the only benefit of using dynamat at all on this mod other than possibly looking shiny, is that just maybe it absorbs a nuance of the high frequencies from bouncing around inside the earcup more than once - that is, some of the high frequencies that echo back towards the driver from the grill may be cancelled out from further ricocheting. Otherwise, the bulk of the reason for this mod making any difference is only because of the butyl, so I wouldn't be surprised if someone could get more/less the exact result with butyl only.
I would have tried the coin-hole mod but was a bit hesitant since I'm sensitive to certain high frequencies. Maybe if I could find a cheap replacement earpad I would give it a go, as a backup in case I didn't like the result.
Overall I'm happy with the result and it is definitely somewhat more clear and punchy than before, but I wouldn't go as far as saying mine sounds akin to an HD '850' - maybe more like HD '690' in terms of difference from before. I recall a heavily burned-in HD-600 that I recently tried on this same system was clearer and punchier, with what seemed an even larger soundstage. I've currently got around 2,000 hours on my 650's at medium volume levels. Maybe I need to burn mine in at louder volumes in order to reach that next plateau of performance.
