Rhod, I am currently away from all of my ref's, and will remain so for the rest of the month. When I get access to a local university library, and Beistien & CAS I can find it for you.
I seem to recall them mentioning that electron injecting substituents maintained or (rarely) increased potency, and electron withdrawing substituents usually decresed potency (especially -NO2). Electron density at the aromatic ring is also important along with even electron distrobution, and unreactivity. Replacment of the benzene ring with similar aromatic heterocycles resulted in reduced potency. The furan, thiophene, pyrrole, pyridine, and pyrimidine analouges were explored.