Some thoughts:
If pots are involved, it seems unlikely the channels will be matched. Maybe in good designs with the pots all the way down.
Lab/instrumentation pre-amps are probably very close and have test/calibration results.
Temp changes tend to cause drift/variation. Some designs have thermal compensation. What tolerance are the internal components? My RMOD pre-amp uses Vishay resistors. They're expensive, large, and hard to package. Many good pre-amp designs separate the left/right signal path... But does one side tend to warm up more than the other due to proximity to heat sources like the power supply?
Grace ships a custom calibration sheet with each v3. I'm sure it isn't instrumentation accurate but it is a good sanity check.
If you want to compare pre-amp channel balance as a baseline, I'd suggest using line out from something (sine wave, white noise, etc). And as ms suggests, moving the same output to each input.
Since a lot of gear tends to distort at high input levels, especially at low frequencies (even before "clipping"), I'd expect the behavior to be somewhat inconsistent under those circumstances.
What about the awkward question of whether the sine waves being close at 2Khz means the mics will sound the same overall on music?
I have always advocated taking "noise floor" baselines of gear, especially more fragile stuff like mics and cables. That way, if you're wondering "hey, do I hear a buzz in this channel?", you can go back and re-test against your baseline to see if it is something new, etc. It also gives a sanity check in deciding whether you need to send mics to Germany for service, etc.