Center imaging problems

"If it still favors one side, I'd check to see if the slightly louder side has less reflective surfaces on the side wall."

Or if that side wall is nearer to its speaker.

This should be tested in mono. Can you select mono on your preamp? If not try a mono recording.

Try extending a length of string from the center of your listening seat to the front left speaker. Hold the string firmly and extend it to the right speaker to see if the speakers are the same distance from the listening position. Everyone that uses this method swears by it.

Measuring from the speaker to the front wall for distance is usually a waste of time since most rooms are not square.

There are many things that can cause a slight imbalance.

Are you using tube equipment? Tubes that are not properly matched can cause this problem.

When was the last time you cleaned your connections? Interconnect and speaker connections should be cleaned on a regular basis. Many audiophiles use Caig products. Caig cleaners leave a residue and I prefer a good cleaner such as Radio Shack cleaners that do not leave a film.

Is your system setup symetrical?

Are your speakers too close to side walls? Side wall reflections can cause all kinds of problems.

Do you play your speakers extremely loud for extended periods of time? This can cause perminent damage to some speakers.

This problem nearly drove me crazy. I did everything under the sun, only to find it was the room.