I use DT770's currently at work (250Ohm), a great headphone for ~£100 IMO.
In terms of leakage they've been excellent and certainly leak a lot less than other peoples closed headphones in the office. Like all headphones, if you crank them right up they will leak a little, that is inevitable.
I do amplify mine (fiio e09k) and they are very refined, reasonable soundstage and are quite enjoyable.
I went auditioning headphones over the weeks (everything up to £250 for closed back) that I could get my hands on, the 3 headphones that stand out for me are:
~£100 I'd definitely still rate the DT770's for their ultimate refinement and ability, and there is something really enjoyable about listening to them for long periods of time.
~£140, the Sony MDR-100AAP were very enjoyable, more dynamic, better imaging, but there was a slight unrefined aspect to the mids/highs that shows when comparing to the DT770's. I couldn't find anywhere with the MDR-1A mentioned above, and I've heard mixed reports on if the MDR-100AAP's are better or worse, it seems split.
~£190 Sennheiser Momentum 2 over ears - These had the same dynamic sound as the Sony's, nice soundstage, good bass but also had the beyer refinement thrown in, I was quite impressed.
The rain forest has the Sens down from £270 to £186 at the moment, so I just ordered a pair (will be here tomorrow), the first thing will be to compare to the DT770's directly.