Hi,
Well I have a similar issue and have tried a good few things.
The first to try, if you can, is try another deck into the same phono input you are using for the WTA. If the imbalance is still there it is the most likely the system not the deck.
If it is the deck then trying different settings on the cartridge position and arm bias will have a very small effect.
I would suggest before you go to looking at any cartridge issues change the speaker positions very slightly, either angled in more or less.
You may find that having changed your cartridge you are in fact exciting the room with a frequency that is causing this effect and that to cancel it you can try moving the speakers. Remember that both speakers dont have to be in exactly the same position, one can be angled in more or less than the other and one can even be slightly closer to you.
Try this before looking at the cartridge.
Cheers
John