Well I definitely have the muffled hearing, and tinnitus as my symptoms. Those all started after a flight/vacation. The symptoms got better over a few weeks, and then I went on another flight, after landing the tinnitus and muffled hearing got worse again. I also "feel" like there might be fluid in the ear, but went to a few ENT's who looked inside and said everything looked normal.
I was actually diagnosed with Meniere's disease (inner ear / cochlear problem) because I also had a couple of vertigo episodes during the initial stages of this whole thing.
But since the flying/air pressure seems to be correlated to my problems, not to mention I was smoking a lot during that vacation and I normally don't, I am thinking it might be eustachian tube blockage.