An edit on my above post which was written hastily.
About the Senn. HD 650 compared to some flagships: I liked it better than the Oppo PM3, Audeze LCD X, V-Moda M 100, Fostex TH 900 & EL-8 all for the same reason: Mainly because it seemed to have better detail in the highs and a better soundstage. Both the the Audeze LCD XC and Denon AHD 7000 excelled in the highs, dynamics and refinement, but maybe a bit piercing and not quite as organic overall and a lesser soundstage. Audeze LCD-X (or perhaps it was LCD 2) was a little to warm / flat FR for my tastes but actually blew the lid off the HD 650's soundstage. Alpha Dogs equaled the energy of the 650 but may have had a smidget too much bass and a smidget less impactful dymanics. Senn. HD 580 equaled the energy, soundstage, etc. of the 650 but was a smidget rolled off in the treble. Hifiman HE 400i came remarkably close the the 650 but seemed somewhat 'thin' and boomy with odd sounding mids, smaller soundstage and highs maybe a bit piercing. Others that I have previously tried which I know to be superior than the HD 650 (most notably for even more organic / lifelike sound, full bodied and P.R.A.T.) would be: Hifiman HE-6, Stax SR-07, SR-09 and Sony MDR R10; and I'm not discounting the Beyerdynamic t5p either. And believe me, all these tests were done using multi-thousand dollar amps. This is of course just IMO being used to the HD 650 sound - YMMV depending on your sound preferences, your source-chain and possible mods you may do to the headphones.
Edit: I have yet to stack the AKG K712 but I've heard it's quite comparable to the 650, and I am still trying to quantify the differences between it and the HE 1,000 which is a whole other headphone.
About the Senn. HD 650 compared to some flagships: I liked it better than the Oppo PM3, Audeze LCD X, V-Moda M 100, Fostex TH 900 & EL-8 all for the same reason: Mainly because it seemed to have better detail in the highs and a better soundstage. Both the the Audeze LCD XC and Denon AHD 7000 excelled in the highs, dynamics and refinement, but maybe a bit piercing and not quite as organic overall and a lesser soundstage. Audeze LCD-X (or perhaps it was LCD 2) was a little to warm / flat FR for my tastes but actually blew the lid off the HD 650's soundstage. Alpha Dogs equaled the energy of the 650 but may have had a smidget too much bass and a smidget less impactful dymanics. Senn. HD 580 equaled the energy, soundstage, etc. of the 650 but was a smidget rolled off in the treble. Hifiman HE 400i came remarkably close the the 650 but seemed somewhat 'thin' and boomy with odd sounding mids, smaller soundstage and highs maybe a bit piercing. Others that I have previously tried which I know to be superior than the HD 650 (most notably for even more organic / lifelike sound, full bodied and P.R.A.T.) would be: Hifiman HE-6, Stax SR-07, SR-09 and Sony MDR R10; and I'm not discounting the Beyerdynamic t5p either. And believe me, all these tests were done using multi-thousand dollar amps. This is of course just IMO being used to the HD 650 sound - YMMV depending on your sound preferences, your source-chain and possible mods you may do to the headphones.
Edit: I have yet to stack the AKG K712 but I've heard it's quite comparable to the 650, and I am still trying to quantify the differences between it and the HE 1,000 which is a whole other headphone.