My 1350's returned today, thanks Marv... But something is wrong, there is some channel imbalance and the resale value has been cut in half!  lmao!  What did you do to my headphones?   

Shhhh! But seriously, do you hear what I'm talking about? The bass and mids are lower volume on one channel while the treble stays the same.

So go ahead and tell me why my "test" doesn't tell us anything...

I went ahead and made up a well fitting "adapter" for my SPL meter and ran a series of frequency test from 20 to 20000hz and moved from L to R and recorded the results.

The results seemed quite repeatable as I was able to go back and retest several times and get the same relative difference (or no difference in roughly half the cases).  I ran the test at several gain settings and was still able to get the same relative differences in channels.

What I found was that typically the L side was about 1db higher than the right at about 50% of the sample points, more predominantly from 1000 down. 

Above most of the readings were even.  The only exceptions being 12500 and 16000 where the L was 2db higher.  20000 was even.

50hz was the only lower range where saw the L 2 db higher than the R.  200 and 800 hz were exceptions too, where at 200 we saw the R side +1 over the left and at 800 both sides were even.

I also ran pink and white noise, both showed the L being 1bd higher than the right consistently.

So, what does this tell me?  It supports what purrins graph tells us, that there is an imbalance to the left, but, assuming my numbers mean anything, 1-2db seems hard to detect with ears. 

Can I make the bold call that the difference in channel imbalance is small enough to be difficult to detect as a shift to the L, but still may have an overall sonic effect.

Ok, don't shoot the poor mechanical engineer here!  I for one make no claims to have any background in audio engineering, so this little experiment was just me trying to get my head/ears around why I was not hearing an imbalance.

No, actually I think I am hearing that... but it's subtle and I was concerned that I now have a built in bias to hear it.  Voices are easier for me to pick up than the bass, it's ever so tilted to the left.  And while I was trying to hear it I was starting to think it was the recordings, so I went to test tones... Again, subtle, so I thought I'd try and put a more unbiased ear on it.  Just playing around... this is interesting stuff, and while at times I like what I hear from the 1350 there are as many times that I find them a little off, so just playing a little to try and learn.

I didn't notice.  I just heard "off"