Volume issues: iPod touch 4G or headphones?

Whenever I'm listening to music, I have to keep adjusting the volume to maintain the same loudness as it originally was, and the reason for this is because, when I'm listening to music or watching YouTube videos (music) the headphone volumes seems to go lower even though the little circle on the virtual volume slider is in the same place (I usually keep it in the center or close) I have to keep adjusting it to maintain the same volume. It's hard to listen to music when the volume isn't consistent, even though it's pre-adjusted. Any reason for this? Its annoying. Im like listening to music and the volume goes lower around 5-30 seconds in to the song, and later into the song the volume will get a little louder than I originally had it. The headphones I'm using are a pair of expensive ultrasone Hfi-450 headphones. Do you think the phones are defective? The volume drops in both ears. When they start to go lower, it's very gradually where it's slightly noticeable. It happens in every song. It doesn't always happen, but mostly. I tried re adjusting with the plume rockers to see if it would help, same thing. The headphones have a special technology called s logic, which is a surround sound feature where's the speakers (drivers) are positioned lower to produce a surround effect and with it has 98 percent less sound pressure even at higher volumes I believe from the off center speakers. So what do you think is wrong?