Thanks to all of the great posts and guidance here in this thread I was able to successfully do my first headphone mod ever! I was a little nervous since the HD650's are the best cans I personally own, but with the detailed instructions in this thread I was able to do it - and boy they sound great! I agree with purr1n,  with the mod they sound crisper, more lively, less over damped. And maybe it is my imagination, but it seems the sound stage is a bit better as well.

I want to thank purr1n, hans, ultrabike (great video), and JoelT for all of your help!

Below are some pics of my HD650's modded going by this thread's instructions - Please note the first 2 pics are not mine, I found them online to help others who want to mod their HD650's on how to use the tweezers method to remove the foam from the back of the driver behind the grills - the rest of the pics are mine (sorry my camera sucks)

How to remove the foam from the back of the driver with the tweezers method (not my pics, for illustration only)





OK these are my modded pics of the HD650 (again sorry about poor picture quality:

Behind grill and drivers with foam removed:



Using the "Quarter Method to cut out the foam exposing the driver (I didn't do so well, but they still sound good! ANd replacement foam driver covers on eBay are only about $2.50 a pair if I need new ones:



And what they look like assembled from inside the ear-cup:




Thank you again to all who provided extremely useful information and tips to mod the HD650, I love the way these sound even more now, and purr1n was right on with his impressions!

Let me know what you guys think, I know that the holes I cut over the drivers inside the ear-cup are not perfect, but this is my very first time disassembling any headphone or modding any headphone and I was really nervous as these are the best headphones I currently own. Does everything look ok to you guys? I know the holes over the drivers aren't perfect, but did I do it right?

Looks fine to me. If they sound good, that's what matters. Plus replacement foam is cheap as you said.

The HD650's were one of the first headphones I ever owned.  I spent some time trying to "upgrade" them and find something I liked better.  I went through the Q701, HE-500, T1, HD800.  Eventually I gave up, and just decided to buy an "end game" amp for them which was the Zana Deux SE.  I couldn't be happier.  While I still maintain the HD800's and enjoy them, if I had to chose between the HD650 and HD800 the HD800 would lose out.  I'd image that most people don't pair a $2000 amp with a $300 headphone.  It's really something special though.

Wouldn't surprise me. HD800 doesn't scale much if any on the ZDSE. You should get a ZD Super sometime or have it upgraded to a Super. I hated the ZDSE and made sure to tell Craig every chance I got. Unfortunately, I can't do that anymore. 

The HD650's were one of the first headphones I ever owned.  I spent some time trying to "upgrade" them and find something I liked better.  I went through the Q701, HE-500, T1, HD800.  Eventually I gave up, and just decided to buy an "end game" amp for them which was the Zana Deux SE.  I couldn't be happier.  While I still maintain the HD800's and enjoy them, if I had to chose between the HD650 and HD800 the HD800 would lose out.  I'd image that most people don't pair a $2000 amp with a $300 headphone.  It's really something special though.

Not sure where the Ragnarok fits in the scheme of things as hype seems to have died down, but both the HD650 and HD800 sounds magnificent through it. I love the HD650, but it's no contest through my system. The clarity and precision imaging of the HD800 is in another league and I'd be loathe to part with it to only slum it with the HD650.