I been listening for this specific problem. I have a set of MDR-V6's and also a set of high end shure inear monitors (for recording, not for the listening). I've listened to base intensive tracks to see if I could determine a frequency range that would be causing a problem. With inears, I always have to watch the base response because it seems to overpowering with every set of inears I've ever used. The shure monitors (SCL5) do a good job of no overpowering me here as do the V6's. I dont know, I would say that if its causing you alot of headaches it could probably be a specific frequency that your ear is sensative to. I've noticed this in my car stereo that installed w/in the last 2 years. The thing gives me a headache and its native EQ sucks... it was 75 dollars... and I was broke... and it had AUX in and i wanted quick fix. I'm sure that ive more than paid the price for my poor, cheap decision.
All this to say, it seems really unit-specific. Other people have been complaining about humming in their audio. I dont have any. I have hooked the ipod up to a nice pair of Studio Monitors and they only hiss I hear is from the orig. recordings.
To add to your note about studios overdriving their songs. In the last ten years I have noticed this more and more. It's really annoying that it seems to be the norm. It's all those trance music people out there that are screwing us all (jk!).
Also, since the ipt has a nice LCD I would guess alot of people are looking at it and getting headaches while they listen to their trance music...hehe. Little LCD's always give me headaches.
Anyways, good luck with finding EQ nirvana
Arisian