I downgraded to 2.0.1 and the problem still exists. That is why i think the upgrade to 2.5.1 messed up the ANC hardware wise, so I went to Bose customer service centre to fix it, though they gave me a new one instead.   Really suggest you guys do the same, if your qc35ii is still under warranty. ... View more

I went to my local Bose Support Centre and they exchanged my old one with a brand new qc35ii. It is on 2.0.1. Never upgraded. The ANC is much better now, back to what it used to be. The customer service officer tested and compared my faulty one with another QC35ii, he didn't notice much dfference. However i really notice the difference, the new one has the cabin pressure feeling and the faulty one has not.   I suggest you do the same, change for a new one asap.     ... View more

I also downgraded to firmware 2.1.3 and 2.0.1. The ANC is still terrible. I think the update to 2.5.1 broke something in my headphone, which result in reduced ANC. ... View more