After trying the bass with the new varnish and the bass with the old nitrocellulose varnish, I bought the one with the nitrocellulose varnish.
The choice was difficult, but after spending two hours with the luthier comparing the instruments, I ultimately preferred the sound of the bass which had the nitrocellulose varnish.
I hope you're not concluding that the finish is a significant factor in the sound.