As I was perusing one of my favorite audio hobbyist sites, AudioCircle, I saw this message. The poster had bought a pair of used headphones and at first he wasn't too thrilled with the sound. Then he made some adjustments to the ear pads and suddenly the earphones had bass, lots of bass.
I recently had a similar experience with a pair of pretty expensive over-ear headphones (the review is in progress, so I'm not naming names) which fit so badly at first that the top part of earphone was ½" away from my head. There was no seal around my ears, and there was no bass.
When I pushed on each side of the headphone so they would make a decent seal around my ears the sound of these cans changed from no bass to MEGA BASS. I could actually "dial-in" the bass by pushing just the right amount. Obviously the comfort level of the phones wasn't going to be quite what the manufacturer intended if I had to go around pushing in the headphones all day, so I attempted to adjust the fit.
After several minutes of judicious and artful bending of the main rods in the headpiece, I got the earphones to the point that they were sitting properly on my ears and pushing in on the sides didn't cause a massive shift in harmonic balance. Victory!
Now I had bass, big bodacious bass, and the earphones were also much more comfortable. And how big a change was it? The earphones went form "I hate these..." to "I'm in love..." merely by the quality of their fit.
So the next time you go headphone shopping, maybe you should be looking for the headphone that fits you best, instead of merely the best headphone...