I was in your boat and never really found a great white that didn't show the seam allowance. I solved the problem by purchasing very white bleached batting (I ended up with an 80/20 blend but would have preferred 100% cotton). It truly made a huge difference.
I am working on my third quilt with white as a background and using Kona Cotton color "snow" since it provides a little softness, not bright white, but looks white against the other fabric.
Good luck and happy quilting!