I don't have data that includes WLS prior to 1975, but in that year Peoria listening was only 30% to FM. WIRL had an 18 share of teens to a 10 share for WLS. Overall, WIRL had a 16.6 share and WLS had a 5.7.
I saw some 1972 rankings, and in every book, WIRL beat WLS in Peoria by about two to one.
WIRL was a pretty good Top 40 earlier on, but its biggest problem was lack of night coverage of the full metro. On programming alone, it stood up to WLS quite well. The areas that WLS did well in were places like Ludington, MI or Coldwater, MI, where there was no good day and night Top 40. Places as small as Muskegon and Peoria and Rockford and Grand Rapids actually had very good Top 40 stations with lots of local promotions and recognizable DJs.
For grins, I looked at New Haven, CT in the early 70's. Local... and signal impaired... WAVZ-1300 generally beat the 50 kw NYC monster at 77 (insert chime here) by two to one. Your point is well proven.