I recently sprung for new ear pads and headband on my 580's, mine were VERY old and tattered. I think it was a good $70 for all of it, and it was SO worth it.
On to another 10 years with my 580s.
Ditto. My search was over for the best open dynamic circumaural head-phone after hearing the HD580 in 1997. Almost twenty years later, my musical tastes and hearing has changed while the HD580 haven't changed a bit - except some wear and tear change-outs.
AFAIK the 650 always used selected matched pairs of 600 drivers.
I had an original pair of HD650 from 2003 and they most definitely had a different sonic characteristic from my HD580 Jubilee in the bass frequencies. The HD650 manufactured recently have the same sound signature with some housing tweaks.
On your other point, newer mid-tier motherboards are starting to appear with actual op-amps (some even socketed) to drive high impedance headphones.
The GA-Z97MX-Gaming 5 uses a TI/Burr-Brown OPA2134 coupled with a Realtec ALC1150. I've heard it and the sound on my HD600 rivals that of the ODAC+O2. The motherboard also has separate traces and grounds for analog/digital and uses Nichicon audio caps. It even has onboard USB power filtration for usb dacs. lol
My ASUS Z97-Deluxe is also built with sound quality in mind, but possesses a TI RC4580 instead. Still sounds pretty good. These are great times to be a true objective audiophile with a subjective viewpoint to fill in the stray data.