If you are having ringing in your ears after 40 min you are a) playing the music ridiculously loud and b) permanently damaging your hearing. Rule of thumb: ringing ears = temporary and over time permanent hearing loss.
Back the sound down a lot: you will adjust to the lower levels and enjoy a lifetime of headphones instead of a few years.
EDIT: Guess that explains why they are fatiguing...
So I went out and got a SPL meter and setup measuring as recommended in other threads. I don't believe I have been listening to music very loud. The range of the levels go from 75-85dB with an average around 83dB when using the C weighting measurement method. If I use the A weighting method then everything is about 10dB lower. Very, very rarely will it jump up to 87dB when I get a loud song and feel like listening louder than usual. Songs like "Woman" by Wolfmother, "How I Could Just Kill a Man" by Rage Against the Machine, "Discipline" by Nine Inch Nails and "Calabria 2007" by Enur/Natasja are the louder ones that get close to 85dB at normal volume. Chili Peppers music which I listen to most max out around 83dB when listening loud, 80db normally though. The Steve Hoffman "Stadium Arcadium" master is around 78-80dB. Average of 83 is not terrible, right? The HD650's sound best at that level to me.
*Note: Music was played using replaygain (per-track).
Below are two pics of my setup. Note the volume on the amp at how much a slight turn can make so much a difference on the dB level.