WRKO can be a powerhouse into Maine and, I'd bet it comes in well in Canada's Maritime Provinces too. It is not uncommon for coastal cities to be served by 50 kw directional "blowtorches" that are located inland from the city and that beam over it. This serves to reduce interference with inland stations.
In WRKO's case, stations in Baltimore (WCBM), Toronto (CFTR), Binghamton, NY (WINR) and even San Francisco (KNBR) need to be protected. WEEI is directional eastward to protect signals in Cleveland and Denver (among others). And, don't forget WCRN 830 from Worcester, which is another 50 kw "Red Sox" signal that is directional to the east to protect signals in places like Minneapolis (WCCO) and Reading, PA (WEEU).
So, the net result is that the 'Sox are available all over the dial in Halifax, St. John, pretty much all of Maine, and to ships at sea - but are really limited anywhere west of the New York border.
The only Red Sox affiliate that can be heard on the radio to the west of New England is WTIC 1080 - which is where I now hear the games in PA. That station should be available at night (at varying signal quality) from roughly Atlanta to Chicago and eastward. Farther west, you'd probably get KRLD Dallas on 1080 instead.