I've been also looking for a good set of headphones with great attenuation, and Bose's don't seem to be the option. According to some study I found (
Attenuation Values of a Noise-Canceling Headphone | June 2008 | The Hearing Industry Resource) Bose canceling headphones offer around 13.3 dB of attenuation. The attenuation level is not provided by Bose on their headphone specs or by any other retailer I could find, which I found rather suspicious.
There are other manufacturers claiming to have achieved around 29 (Passive) to 30 (Active) dB of reduction. Personally I prefer passive because it will also prevent bleed from headphones to mics whereas active is designed as a one-way attenuation system.
The alternatives I've found are
1. Direct Sound EX-29 Extreme Isolation Headphones claim to offer 29 dB of passive isolation in the 80-90 USD range, although some reviews I've read describe them as fairly sounding but cheaply crafted. I contacted the manufacturer and designer a few years ago to ask if I could have a custom made set with detachable cord, and he said he could biuld it, but he could offer no warranty over that set because proper testing would be very time consuming, so I didn't get them in the end.
2. JD Power Sound Super Noise reduction canceling Headphones claim to offer 30 dB of active attenuation in the 200-250 USD price range, but haven't found any reviews on a site I trust. If they actually sound good and can offer the 30 dB of reduction, they might be worth to hear. I've been considering to check up on these.
DP