I can't help much with your question about muslin being too thin, but your concern about pre-washing preventing the crinkled look, I can help with. I pre-wash everything, I've had some scares that were discovered before doing all the work, so I will continue doing this. I recently finished a quilt appliqued with Kona as the background and Hobbs 80/20 as the batting. I hand quilted it. I washed it because it was one of those that took me years and years to finish, so it was dirty. I was surprised how much it shrank with everything being pre-washed and the batting having some poly in it. It definetly has that old fashioned crinkled look.