Something strange with imaging

Yup could easily be the ears and solution maybe a pro ear cleaning.  Eliminate that possibility first.  I recently had a similar problem.   Sound was  suddenly unbalanced and highs distorted.  At first I thought my hifi was maybe snuffing it but a few simple tests established it was mostly my right ear.   Got the past due ear cleaning and voila, heaven once more. 

Dear @wlutke and @mapman and any other aspiring ENTs,

Not ear wax.  That was the first thing I checked!  All other sources including CD, streaming, and reel to reel have perfectly centered imaging through the same preamps. Any other ideas?

Hard to say. I had a similar sounding case recently where I found a particular phono preamp setup I was using to convert to digital had a small hardly audible but measurable difference in output levels left to right. It was simply a minor flaw that may have developed with the device over time. I went to a different setup recently but with same cart and step up transformer, and now things measure equal left and right as they should.

So ideally it’s just a poor connection or combo of connections perhaps but always possible any device involved in signal path from the cartridge on could just be off somewhat left versus right. I’ve seen it happen on multiple occasions over the years with phono cartridges in particular.

It’s also possible more than one device in the chain has some l-r output level difference and they cancel each other one way and augment each other to be heard the other.

Not a good idea to judge channel balance with a stereo recording, because you don’t know what the recording engineer had in mind. Better to use a mono LP and listen for a centered image.

your seemingly conflicting results could reflect differences in stereo recordings.

Image position follows the tweeter, as I found out when one of mine failed.

You’ve established that the issue is somewhere before the phonostage, a dameged cable with a higher capacitance on one channel might be involved but I think it a long shot. What happens if you swap channels at the cartridge tags and the phonostage input? 

Try another cartridge.

How are you setting bias?  If you do end up sending the blackbird back to Soundsmith get them to assess the wear on the stylus and condition of the suspension. This can tell you if you’re getting it close enough.