At the core of that problem a technical issue is that the Supernait 3 seems (at 130 mV sensitivity IIUC) to have maybe up to 20 dB too much gain for when it is set to mid-volume, driving 'speakers with 89 dB SPL sensitivity, from a nominally 2V full scale line-level source (I assume). This has been pointed out by several contributors and accords with my experience of gain line-ups in audio kit.
You have to throw this extra gain away via the volume control. Instead of being centred at maybe 20 dB attenuation (11:15 clock position), it has to provide 40 dB attenuation at about 08:30. A consumer-priced potentiometer will inevitably fare worse there and balance will often drift off.
The solution is to avoid whatever is likely or inevitable rather than just blame the potentiometer. Although I sympathise with the OP's frustration at not being assisted (by his dealer) to a solution, I doubt his strong opinion that the Alps pot is fundamentally to blame. Unless it is actually broken replacing it may or may not be a fix. I have had two of the Alps blue RK271 here (rather fewer than
@Arkless Electronics has seen) and they have both performed well for balance, even down to about the 08:30 position.
If sticking with the Supernait 3, a part of the solution may be to try about 20 dB of attenuation before trying any surgery. The volume pot will then move up to a place where it should be in more consistent balance and where the balance control can be set once for all volume levels. Nominally, attenuation should go between preamp and power amp. But I think this is a 5-pin DIN interface and I don't know of a current product. I would build my own but the OP may not have the know-how. Maybe 20 dB RCA attenuators on an input can be tried. I am surprised the dealer could not help here but I think the OP's frustration, although understandable, may be getting in the way of a solution.
