Sounds like something wacky is going on in there.
Properly set up, you should be able to pour massive amounts of power into the T50RP without any distortion or strangeness in the sound. It sounds like you might have something loose in there that's buzzing. A picture of what you did in there might help, or a description of your mods.
If you haven't done any mods yet, there is something you should try: reseat the cable in the left ear cup. See if that's partially disconnected.
What amp are you using?
Edited by joelpearce - 12/23/11 at 8:48pm