I would like to add my two cents.
2009 HD650 vs 2015 HD650
The old 650's had their pads and headband replaced by genuine sennheiser parts 2 weeks back so both the 650's have similar use on their pads.
I still have both the 650's and the 598
I know for a fact that Senn has changed the tuning on the HD650 driver over the years even on the Silver driver versions.
I'm on my 2nd HD650, the 1st one was bought in 2009. It came in a silver case which came inside a silver corrugated paper box. The 2009 HD 650 is still going strong sonically, just beat up cosmetically. This 2009 HD650 had the silver drivers and paired with the HA 160D I didn't find the sound to be veiled or muddy. Burson's design brief for the HA160 was to drive the HD650 effortlessly since it was one of the most difficult to drive cans on the market so if the HA160 could drive the 650 effortlessly other cans available at the time would pose no problem.
This 2009 HD 650 out of the box had the vice like grip 650's are known to have and to loosen it I had to put in on a 10 inch wide bookshelf speaker(treating the speaker like a head) for weeks. This helped reduce the clamping force and the headphone became very comfortable.
Now this HD 650 had AWESOME bass weight and impact just like what Mike from headfonia says. The bass was weighty and detailed with very good extension. I also bought a HD598 in 2010 and compared to that the 2009 model HD650 was way ahead, more air, a presentation that was pulled further back, better timbre and the bass slam, weight and impact was waaay ahead of the 598.
Fast Forward to 2015
I really like the HD650 and since the old one was looking pretty beat up I decided to go for a new 650.
Going by my previous experience I was prepared to go through to vice like grip loosening phase but.....
The November 2015 manufactured HD650 had a very light clamping force out of the box which was at par with the 2009 HD650 after the bookshelf speaker loosening period.
2nd difference was the box was now black and the headphone case is dark grey.
BUT THE BIGGEST DIFFERENCE B/W THE TWO WAS THE SOUND
The 2015 HD650 had no bass weight(even less than my 2010 HD 598) and had lost its laid back character, the sound has a forward presentation plus the treble is boosted and sounds like when 12kHz-16kHz region is given a+3.5dB boost. I still have the HA 160D and compared both the headphones on the same setup. Headphone was burned in for 200+hours.
The timbre, instrument separation, bass extension and detail retrieval is same but the tonality is different and the bass weight is absent. Gone is the warm laid back presentation. Sennheiser HD650 just pulled an Audeze.
2009 HD650 is a laid back warm sounding can with AWESOME bass weight and slam when needed but the 2015 HD650 is completely lacking in this department, its neutral sounding with boosted highs.
My old 650 has thousands of hours on it and the new one has less than 300 hours on it. I'll keep using the 2015 HD650 and report if the bass opens, if not Sennheiser has once again covertly changed the sound tuning of the HD650. These difference were also there when the cans were used with a Burson Conductor Virtuoso.
EDIT: 23/JAN/2016
The new 650 now has 1200+ hours of play time on it. One thing is for sure the HD650 takes ages to run in. If you don't have another HD650 for reference you will find that 350-400 hours of run in time is sufficient.
The 2015 HD650 started sounding like the 2009 HD650 around the 800-850 hour mark.
Changes to the sound
Out of the box the sound was lean,cool, bright and forward compared to my 2009 HD650. Post run in period, the 2015 HD650 has the same smooth, organic, slightly warm sound as the 2009 HD 650.
Out of the box the treble was hot and a too aggressive compared to my old 650 but that issue is no longer there.
Bass impact/punch/kick is greatly improved.
One thing that is still missing is the bass weight. It has improved but its still lacking a bit compared to the 2009 HD650.
