as far as the amp goes, the O2 looks like it would be great except it has an 3.5mm in, which doesn't sound great to me, but it would certainly do.
As far as refurb headphones, I'm not entirely sure I would trust that, and the only thing that really put me off from the DT770/990 was the different options. I'm in a family house, so I'm not sure whether I would want the open design. Any indication on how the sound leaks from the 990s, and would I want the 250 OHM model?