Transistors

[Murphyv310]

Correct me if I'm wrong but I would doubt that IF frequencies were worked out by trial and error. Early superhet radios had lower IF frequencies often around 120-170 kHz, this was done as the valves of the time performed better at lower frequencies. Later on with a higher frequency IF the valves had improved and it was also simpler and cheaper to wind the higher frequency coils. Also don't forget that higher quality and communication sets will have two frequency changers and dual IF, not to be confused with AM & FM sets.