At around $200 you going to start finding headphones with a more balanced sound, then the bass heavy ATH-M50s.What sources will you be plugging the headphones into?Does it matter if the headphones are open or closed?
So your thinking of getting the Sennheiser HD650s and using them with your iPod?If you have an older iPod that comes with the LOD port, you can get the FiiO E11K headphone amplifier ($60) and connect it to the iPod's LOD port.Or just get a Digital Audio Player that comes with a decent built in headphone amplifier (that can also play Apple audio files)FiiO X3 or FiiO X5?
As you already have the Scarlett Focusrite 2i2, might as well get some headphones and try them with the 2i2.Worse that could happen is you find the 2i2 is not good enough and have to buy a new DAC/Amp. I like to think the Sennheiser HD558s ($80-$120) are great bang for the buck headphones.
You listed the 900X, assuming talking about the closed version?I would take the AD900X or A900X, over the HD558s.Maybe the open version (AD900X) over the closed version (A900X). AmplifiersFiiO E11KSchiit Magni, $99O2 (Objective 2), $130
For headphone like the Sennheiser HD558s, I would not spend any more then what a E10K ($75) would cost.Otherwise it might better to sell off the HD558 and put all your audio budget into new headphones.Are you using the E10 or E10K?