There's a good chance that the difference you hear is caused by asymmetry in the room acoustics and not the electronics or speakers. In a good setup, a mono source should appear as a phantom image in between the two speakers.
The way to check that is to switch the left speaker to the right and see if the situation reverses. You can narrow things down by switching L/R on the source, then the preamp, then the amp as well as at the speakers.
If room acoustics, you can experiment with speaker placement and room treatment.
The way to check that is to switch the left speaker to the right and see if the situation reverses. You can narrow things down by switching L/R on the source, then the preamp, then the amp as well as at the speakers.
If room acoustics, you can experiment with speaker placement and room treatment.