Just keep the Sundara and don't buy new headphones unless you find a crazy deal on a used pair. But it is nice to have a few headphones for different use cases, but in you situation it does make sense to sell the Sennheiser if you have better refrence headphones. The Sundara are great and are called by almost everyone as the best headphones around $300. So I don't think you need to upgrade from them, and plus the upgrade is barely going to be noticeable.