Hi Blindjim,
The AKG K702s plug in to a standard headphone socket (1/4" or 1/8"), not the speaker terminals of an amp. They are open-backed but ambient room noise is not an issue as open backed really means sound gets out rather than sound getting in - unless you are listening in a very loud environment. In terms of sound quality, it seems universally accepted that open-backed give better sound quality than closed 'phones, though I haven't listened myself to test this. The K702s are essentially the same as the K701s but with a detatchable cord which I think is useful and enables you to upgrade the cable without having the phones modded - or even just get a longer (or shorter) cable.
I'm very impressed with the sound this combo (with Audio-gd amp) is producing but have not experienced many other combinations to compare, certainly not at this budget.