Bamboo 150 - faster than the 220 and MUCH more stone. Faster at removing scratches from the Atoma 140. Once the scratch pattern from the Atoma is 'converted' to a stone scratch pattern, you might find - depending on the knife being sharpened - that you may be able to skip the 500 too as it leaves a nice finish. Very slow dishing too.
And depending on the knife too you may not need to use the Atoma too much either
Ok I'll admit it - my 150 Bamboo is my favorite low grit stone.