I have to agree that this issue is in the analogue domain. However, if that is the case, I find it hard to believe your statements that the issue does not happen when:
- Using the turntable direct into the amp
- Streaming without using Roon.
You say you have already swapped the speakers left to right (but retained their positioning) and the problem remained with an imbalance to the left (-3dB on the left fixed it).
You also say that the issue was seen with a previous amplifier. That eliminates the possibility of the amplifier being the cause - it is highly unlikely that you received two amplifiers from different manufacturers with the same fault.
This only leaves speaker position and room acoustics as the cause as @wklie has said. But again, if that was the case, you would hear the same misplaced soundstage no matter which source - Roon, other streaming/digital or analogue, was used.