IMHO, the only headphone that offers more isolation than the Sennheiser HD25-1 II (or Amperior) is the Etymotic ER-4. Seriously, as far as headphones are concerned, the HD25-1 II's offer the highest dB differential of any headphone: http://headwize.com/?page_id=550
The HD25 breaks 140dB. Very few headphones in that reference even break 130dB.
I've been reading a lot about the Etymotic ER-4 series.To get the best isolation is it necessary to insert them deep inside your ears or is it sufficient just to use the foam tips?