I have a slight hearing imbalance where some higher frequencies in my left ear are not as strong as my right ear. I only noticed this when I bought a decent tube headphone amp and headphones.
If you’re running single ended connections, I would try the Douk Audio (Nobsound) balance control. It’s built using Yamaha control board slide controls. I noticed no degradation in sound in my headphone setup.
For $150, it worked and is built better than I expected. It has a very fine adjustment range. You can find them on the bay searching “HiFi Stereo Passive Preamp Lossless Audio Preamplifier with Volume Control”