Thanks for this. I’ve tried various thumb positions but they haven’t helped. The problem is the pressure between the flesh and the edge of the nail along the left side of the thumb. The rather sharp edge of the thumb-hole seems to aggravate this.
My Aulos bass seems particularly critical over the thumb position playing a top D when played ‘cold’, or from a note in the lower register. When approached from the C just below there’s no problem at all!
Would it be detrimental to smooth the edges of the thumb-hole slightly?