I would recommend using crusher run/screenings - about 4" of it, so if you have your loam taken down an extra 3-4" (if final top rail height is important to you, otherwise just build on top of the loam) and bring in enough screenings to create a 3-4' wide circle under your walls. It compacts to a hard base using water and a plate compactor and can be levelled precisely.
Then use 3-4" of sand inside the pool for your base under the liner - I sloped mine to gain about 8" of extra depth in the center because my pool and liner was big enough to accomodate that.