Channel Imbalance - HELP

It has finally got the better of me.

Ever since my first high-end system bought 15 years ago, I got it home and the left channel was louder than the right. I traced it to the amplifier, took it back to the dealer and funnily enough it was perfect at his place. He was good enough to exchange it for a new one anyway, but when I got it home, exactly the same thing!

I swept it under the carpet and just enjoyed the music. A few years later (with a whole new set of components) it was annoying me again, then I had an issue with one of my speakers and blamed it on that - I got rid of those too.

I was in denial.

Now, 15 years later with not one single component the same as in my system even only five years ago, I did some scientific measurements and sure enough, the right channel is 2dB down on white noise!!!

I did some swapped the L and R channels at the preamp output so the signal was going through DAC and preamp correctly, then swapped for power amp and speakers. You know what this did? - evened out the whole mess! Indicating to me that it is a progressive thing through each component.

What's going on??

Sorry for the long post, but I've had enough!!
Wow. Facinating story. Not a clue what might be causing your imbalance. What are scientific measurements anyway?
"Scientific" measurements are using white noise and a calibrated SPL metre.
If I understand your reversing leads correctly, it points to your amp or speakers as the culprit. Measuring output at amp outs with DMM is a more precise way of determining channel imbalance.
I don't own a DMM and I wouldn't know how to measure balance that way anyway.

All I know is that reversing the preamp outputs didn't reverse the issue, it evened it out!

I'm consused!
I had this one time and it ended up being a room problem. My left speaker was always lower in volume. I did the swap thing and it went away except now the imaging wasn't right. I even had my ears tested!
I came to the conclusion, when I swapped things, it caused the signal to be dispersed differently eliminating the problem by somehow taking the room out of the picture. (Using one of the new receiver audio setups (Denon 4808ci) it too had shown the left channel down about 3 db and out of phase.)
I finally played around with acoustic treatments and speaker positioning until I got it very close. (Now it still happens from time to time but it is the disc to blame. It's surprising how imbalanced some discs are.)
First thing that came to my mind was, maybe your left ear is bigger than the right ear?

But on serious note, maybe it's room acoustics?
Are your walls are made of different materials? Room is asymmetrical?
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I have a similar problem, but I can also see the difference on the meters on my MC2205. I'm thinking it's a problem with my MX130 preamp. I guesss I will try switching the cables and see if the problem reverses.