Depending on how new your pair is, you can put the rear foam back in near the drivers. I've taken the foam out of an older style driver HD580 and that ended up being completely irreversible. However when I took the rear foam out my newer pair (silver drivers) of HD600's and HD650's, it came out nice and easy. The older ones were glued in there while the newer ones were just placed in there. With the older glued in ones, the foam was nearly impossible to take out without most of it getting stuck to the frame.
Also just a few interesting notes:
-The HD650 had a thicker rear foam than the HD600.
-HD650 pads are different from the HD600. They are thicker and less stiff than the HD600 pads. I'm not sure if sennheiser sells different earpads for each headphone but I was only able to find the HD600 style pads for sale.
What issue/problem are you trying to solve with these mods?
I found these cans to be quite amp dependent sound wise, so if you think there's a veil or that they're too laid back, you may want to try a hybrid or SS amp of sufficient power. Doing irreverbsible mods on a $300-400 set of cans sounds like a recipe for possible regret. On a $30 headset, fine, take some chances. But on HD650's, I'd be careful and check into other options first.
The HD650's are an open headphone so of course a wooden enclosure would improve isolation. Also instead of doing the coin mod, why not try taking out the foam first? You can replace the foam if you don't like the sound but the "coin" mod is permanent.
Just tried out taking out the front pads, and it makes a huge difference. Not sure if it's positive yet... I definitely hear more air and highs that seemed to be veiled... But that improvement came with a boost between 7-10kHz that I don't think it needed, and I'm sure it'll get my ears fatigued after a while. I'll steal my gf panties later and try that mod