Gents, remember sound reacts to everything in your room. An acoustically perfect room would have to be perfectly symmetrical. Just as Owl's Warder stated the sound difference between blinds up/down. In my experience, a single volume control generally result in one channel being very slightly louder than the other. This slight difference is so slight that is's practically unnoticeable.

Speaker placement will also affect the volume slightly. I'm sure some of you may have heard of SPL (Sound Pressure Level). This is measured in db/distance (usually in meters ie: 93db/1 meter or 93db at 1 meter). So, if your speaker placement is not symetrical (a wall closer to left speaker or vice versa, sound will surely be different). If you are that critical of your system, the industry do sell audio accessory which helps deaden or attenuate those affects. Of course you can do what Owl's Warder did... by moving his left speaker 3 inches... made the difference.

So, there you have it.

Hope this helps.