Can I convert stereo to mono for a phono input

John_Tracy, that is a small difference in output for a significant difference in sweet spot. Mine is around 2 feet to the left, in my estimation. I'm curious whether most balance/volume controls provide that high resolution of an adjustment. I would guess 0.5 db is the lowest.

My experience has been that 0.25dB steps for a balance control is about as "wide" as one would want to go. My DIY line stage also doesn't have a balance control. So I provided my DIY power amps with input attenuators that can reduce their gain by ~2dB in 0.25dB steps.

 Wow! I think I just found a good way to electronically evaluate my cross talk. There is a free program called ispectrum available for the Mac, that provides an oscilloscope with a band pass. I can use my test record with test tones for left and right channels individually  and measure the output for each channel for each individual track.  Input to my Mac Mini can be either through the microphone input, or through my USB analog to digital converter. In either case, the oscilloscope measures both channels, stereo.
Based on prior posts, I doubt I will be able to counter at my channel imbalance this way. Perhaps the anti-skating is off a bit and maybe playing with that will help? 

Another interesting observation about setting channel balance.  I'm looking at the meters on vinyl studio to judge channel balance.  I started with a 1khz -14db test tone on the Cardas Test Record.  It showed a weak left channel.  Antiskating didn't help here at all.  Then I used the Hifi News Test Record with a -20db pink noise channel balance track, and the channels seem balanced in the meters.  Then a mono Ella recording and the left channel is just a tiny bit lower than the right, nothing really significant.  Which to believe?  It seems like my calibration sources are imperfect, particularly the Cardas. How to set azimuth if the source is not truly balanced b/t channels…  Not impossible, but adds a bit of additional variability, no?