hi
i recently bought the beyerdynamic dt-770 M,
they claim that the headphones have an isolation about 35db, but ill doubt that they do, because i got an ear protector for people who are running around the airfield on an airport, and they isolate about 40db, and these are so much more isolating!!
but for your purpose they should do well, the sound is also very good in my opinion, i only use the akg 240 too, and as sound reference they sound a bit more direct and have more clarity.
what i like about the dt-770 M is that you an really turn them up very loud, and nearly dont hear anything outside the headphone, i will mainly use it to record vocals so that there will be no spill from the headphone to the mic.
after all, the dt770 M is quite expensive (imho) and the vic frith should have a similar isolation but is much cheaper
cheers
exoslime