As suggested by Newbee, swap the tonearm cables into your preamp and see if the imaging imbalance shifts to the other channel. If it does shift, that could suggest its a hardware problem or a speaker alignment issue. If it does shift, fiddle with speaker position (toe-in or move one slightly forward or back.) If it does not shift, it could be a TT/cartridge issue, but I'd also look to possible acoustic issues in your room as suggest by Wwshull. If the latter, read on.
If one has stable/symmetrical imaging to start with, its not that hard to cause anomolies at various frequencies by introducing or changing reflective or absorbtive surfaces in the room. Walls jutting out, furniture, ceilings - all sorts of things - could be a cause of a certain range of reflected frequencies stacking up in phase with those coming directly from the speakers - sometimes called a comb filter (iirc). The result can be an over emphasis on sounds in a particular frequency range and that can cause the perception of an image shift.
You can find first reflection points by sitting in the listening position and have someone move a mirror along the side walls. Mark the spots where you see the tweeter of
a speaker in the mirror - typically there can be two spots on each side wall, one for each speaker. Lay 2-4 bath towels on top of one another and drape that over a big piece of cardboard (or something to hold the towels in place.) Towels should absorb the highs much more so than the lows. Position them to cover the reflection points marked on the walls and listen again. Ideally you'd do this on both sides of the room at the same time. Try moving the towels around the room, including behind your listening position if possible. If the area to the side of one speaker is open and the other speaker has a wall beside it, try putting the towels on the wall to match the open area on the opposite side. You should be able to modulate the imaging in this way - look for the spot where the imbalance you're hearing goes away.
Tim
If one has stable/symmetrical imaging to start with, its not that hard to cause anomolies at various frequencies by introducing or changing reflective or absorbtive surfaces in the room. Walls jutting out, furniture, ceilings - all sorts of things - could be a cause of a certain range of reflected frequencies stacking up in phase with those coming directly from the speakers - sometimes called a comb filter (iirc). The result can be an over emphasis on sounds in a particular frequency range and that can cause the perception of an image shift.
You can find first reflection points by sitting in the listening position and have someone move a mirror along the side walls. Mark the spots where you see the tweeter of
a speaker in the mirror - typically there can be two spots on each side wall, one for each speaker. Lay 2-4 bath towels on top of one another and drape that over a big piece of cardboard (or something to hold the towels in place.) Towels should absorb the highs much more so than the lows. Position them to cover the reflection points marked on the walls and listen again. Ideally you'd do this on both sides of the room at the same time. Try moving the towels around the room, including behind your listening position if possible. If the area to the side of one speaker is open and the other speaker has a wall beside it, try putting the towels on the wall to match the open area on the opposite side. You should be able to modulate the imaging in this way - look for the spot where the imbalance you're hearing goes away.
Tim