The topic of noise-cancelling headphones was discussed in Pax&SLF a while back from a pax point of view,
Noise cancelling headphones - again but some professional pilots also contributed comments about comfort when flying long-haul. The A20 wasn't specifically mentioned but the QC25 and QC35 were. Neither are particularly expensive. I went for the B&Ws but I'm just a pax and audio quality is my main concern; the Bose's are preferred by pilots from a comfort point of view.