well… there are at least three other Peter Evanses out there in the music world…
Trumpeter
Guitarist
Britten Scholar
and many other non-music Peter Evanses…
Australian Chef
Writer
and then some Peter J. Evanses on twitter (the 1st is kind of spooky, actually…)…
http://twitter.com/evanspeterj
https://twitter.com/thepeterjevans
…so, the short answer is: ‘theorist’ is both unclaimed and applicable… and unique!