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.
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I use a pair of 650's which sound great with either my desktop or my laptop using a couple of Headroom's portable components:
The MicroDAC http://www.headroomelectronics.com/products/micro-line/the-headroom-micro-dac/
. . . and the Headroom Cosmic Headphone Amplifier http://www.enjoythemusic.com/magazine/equipment/0402/headroomsennheiser.htm
These are earlier models, this is the current package:
Sort of expensive, but if you already have the 650's, worth it. And besides, just because you're 'at work', you may still be alone with no one to disturb ;--) I'd take the Sennies over in ear's anytime.