BOSE quiet comfort 25

Do they work OK using the same cable (less the adapter) with a normal audio source?

Did you have more than one adapter?

The "very quiet" effect is typical of the headphones having the left and right audio signals but no common ground; you only hear the difference between the channels.

How many contacts do the plugs on the Delta ones have? I've seen noise cancelling headphones on some airlines in the past, that have more contacts on the plugs.