Why sound is not balanced?

I have Adcom GFA-5500 power amplifier for yeas to power B&W 801s. Always the left speaker sounds louder than the right.
What would be the problem?
Thank you.
To determine if the preamp is the problem, switch the left input for the right and vice versa. To determine if the amp is the problem, switch the right leads for the left leads. To determine if the speakers are the problem, switch them, placing the current left on the right and right on the left. If the problem persists after these tests, the answer lies with your room. If the room is not symmetrical, identical on left and right because of an odd shape, this is a likely culprit. If the furnishings are different on right and left, (i.e., a big couch on one side and a window on the other) this will cause an imbalance. If you can't do anything about the room, use your balance control to shift more signal to the right speaker until both sides sound identically loud.
That's a great post by Pzuckerman. A very concise troubleshooting guide.
I found a solution to that problem - I didn't have a balance control. I simply moved my listening chair a few inches to the right. If the imaging specificity gets sharper you're good to go. If sharp imaging gets a bit blurred, look for a different solution. FWIW.
I also recommend Pzuckerman's suggestion as I have a similar problem. I did the same tests and afterwards concluded it must be the room.

Now my room is 18.5 x 15.5 and it is pretty rectangular, but unfortunately I have a huge window on one wall which happens to be the side of the room where the left channel speaker is placed. In addition, the ceiling has dips here and there and the walls are not perfectly level. Bad construction in general. My next step is to try to work with my room treatments to see how best to correct the problem as much as I can.

Adjusting the balance was not helpful in my situation because by the time I found a good setting the center image lost focus.
A possibility. It happened to me. Same thing - left channel louder than right. It turned out that my left ear is better than my right one. Turn around and face away from your speakers and see if the right speaker (behind you) becomes the louder one. Probably not the case, but it tends to be the last thing you might consider :-) Good luck, Dave
I'll echo the above postings about first, turning around at the LP and listening... then proceed with the wire swapping....

THEN not only windows but openings to the room will allow for a suck out to whatever side the opening is, especially if it's close to the speakers. I have two openings, one to either side of the room. Though the one on the left (facing the speakers), is closer to them, so that side has a bit less the impact, that the right side has overall.

Past that there is one other item of note... the player. TT stylus alignment and/or the laser in the CDP could be off.

I say that because I went nuts trying to adjust my room, system, and my sinuses which affected my hearing... and come to find out the laser in my cdp was going out. I've since had it replaced with a new drive/laser sled, and now all is far, far, better. My CDP issue was intermittent so it did drive me nuts for some time. well that and my sinuses.
In my system, I did try swapping transports (then other components), but no improvement. While I have the window on the wall where the left channel speaker is placed, there are also two openings on the wall where the right channel speaker is placed. One towards the rear corner and the other towards the front corner.

One thing that Steve Deckert at Decware advised me to do is to rotate the speakers 180 degrees so they fire in the opposite direction. Then set up my room accordingly and see what happens. This requires a tear down and re-configure of the system and I've been too lazy to try that yet.