That is a great question and I'll try to answer with my limited experience and vocabulary.
I would say a full natural sound that has great separation of each individual instrument, but which can express the overall harmonics that result from the interaction of all the instruments. I guess this statement can be said to be the aim of all good headphones, but I am using it to describe the level of quality I want, I guess. The phones need to accomplish this pretty well.
With a good recording, this results in the least amount of fatigue over time, as my ears don't begin to tire from a less clear articulation of each sound. I think that is what is going on, although I'm not at all sure why something that sounds okay to begin with would sound ordinary and noisy after an hour (similar quality of recording I think, and the same type of music).
The kind of music I can relate this to is The Police, Talking Heads, Beck, some world music by Ayub Ogada, Neil Young, Peter Gabriel, etc.
If we take the track Obiero, sung by Ayub Ogada, and I play a flac version from VLC on my desktop PC through my AKG K401s, the base vocal track is quite flat with little harmony. When it gets layered with an organ, the layers are quite harmonic and with the organ (and some reverb(?) are quite rich and warm(?) in comparison to without. However, I would like a bit more depth and richness across all the range on the track. So that the voice is less flat and has more rumble or depth. So, enhancing the sound to help it be more full but without changing it, so that it doesn't become coloured(?).
I don't know if it is the recording and devices that aren't delivering good sound, or if my ears just tire and have had enough sound for a while because they aren't used to listening to music for such long periods (I haven't listened regularly to music for many years). Listening to movies is no problem.
So, to answer your question, I like good bass and deeper trebble (a richer trebble rather , so that each is very clear but the bass is not fuzzy or dullish, nor is the trebble too bright or lightweight. I would say I like to be in the mid-range with good bass and some great highlights. I think I might tire from too bright a sound. Or, too flat a sound.
I don't think I'm a bass head at all but I do like it to be punchy and full.
I hope this helps, Kim.
Edited by gxtoast - 6/24/13 at 7:36am