This is a fun hearing test I discovered a few years ago, and it can teach people the basics in how they hear their headphones. It can be useful to put certain impressions in perspective, and also give a slightly more common ground to exchange impressions among people (although, as we all know, the first “frequency response” which varies among us is the ear, being it because of age, sensitivity, pathologies, and several other factors…).
In the 1 kHz column, choose a panel about halfway down. Listen to it and check that (1) it is not uncomfortably loud and (2) it is considerably louder than the background noise. Go up or down the column if necessary until these conditions are satisfied. Notice that the dB level chosen is now recorded at the bottom of the chart. Once you have made a choice, this becomes your reference sound.
Try to maintain the perceived volume the same for each column, and you’ll obtain the headphone’s loudness curve to your ears.
Another quite good tone generator is here:
Tone generators can be also used to test if a headphone has good seal on the head of the listener (if the listener has bad seal, the air can escape). Anyway, in general, if no bass imbalance is consistently audible with music, one shouldn’t worry too much to test tones.