As others said, it's vital the volume levels be matched. It's simple to do, yet you can't do that by ear. Just play pink noise at the expected listening volume, measure the SPL with a meter, switch speakers, compare the SPL and that's your correction factor in db.
In reality the correction factor will vary depending on listening level, so to be really exact you'd do that for several listening levels, and apply a different correction factor for each. It's really quick once you're used to it, since virtually all amps have db volume readouts.
Another major factor is the amp should be in "direct stereo" mode with all digital processing disabled. Without this, it's easily possible digital time arrival, EQ, bass management, etc could favor one speaker over the other.
Make certain it's in two-channel direct stereo mode, all digital stuff disabled, no surround speakers or sub configured.
