If I reach back to the early 90s, Spiritualized had some of that atmospheric feel that Sigur Ros can generate. Then there's moments of Godspeed You Black Emperor -- but they get far too comatose and repetetive for me at times. At least Sigur Ros pulls odd sounds that keep you engaged.
A close match to Theivery, IMO, are their Austrian bretheren in Kruder & Dorfmeister. Some really good stuff there.
I like an occasional Portishead song, but I'm not much for Morccheeba.