Headphones at work

There's a few portable amps that put out quite a bit of power. I don't have direct experience to say whether they're enough for your 650's, but you could find input on the headphone-specific forums. Ray Samuels SR71B,and iBasso PB2 Pelican are both balanced portables known for their power output. They will not compete with a desktop headphone amp, however, which is the way I'd go if you don't need portability, but they may come close enough. I tried headphones while working and ultimately found them too bulky and awkward while moving around and occasionally having to remove to interact with others. I found I much prefer IEM's, the better of which are quite good from a portable device not requiring nearly as much power. YMMV. It may help to state your priorities and limitations.

650s are open back so sound "leakage" problem could be a problem.

Good point, Swamp. Another good argument for IEM's which are the best at isolating.