Wow, that looks terrific! I like the way the underlying mesh also shows subdued highlighting here. Excellent!

I was going to suggest an alternative to using a Subtract node - a Gain node, which you could use to increase the contrast in the bump map on either side of a chosen mid-point. But this seems to be working very well, and a Gain node would probably lose some of the subdued highlighting of the mesh.

"If I were a shadow, I know I wouldn't like to be half of what I should be."
Mr Otsuka, the old black tomcat in Kafka on the Shore (Haruki Murakami)