I never really had an issue with balance. What you should do is rip out 3.5mm adapter in the left ear cup, fill that area up and top it dynomat and plaxmate. Make sure the amount is as close to each other as possible on both ear cups. Then do a re-cable and your L/R balance should be fixed.
I know this is not a reversible mod, but the T50RP is only $74 and I plan on keeping it. If you plan on keeping yours and the L/R balance issue bothers you that much, you might as well try it.
After sealing them properly, I still occasionaly experience a dull left side, so I switched them around so they were facing backwards, left is now on the right side. I was still experiencing a dull left sound, I think my left hearing has been damaged or something, headphones are renouned for causing long term loss of hearing. Bad news for me, as I was just about to outlay on some nice gear. Might see a specialist first, speakers may be the only (noisy) option.