Back then, you could say and claim to be anything you wanted. Didn’t make it true, but stations made all kinds of claims.
Back then, most STL’s were via telco and there were “grades” of service. The cheapest was a twisted pair. Then an equalized line to 5kHz, which was the norm. A few of the bigger stations would get equalized lines to 8500 Hz or 10kHz. Most stations, and all network lines, were equalized to 5kHz. If a station had their studio and transmitter at the same site, they could have better response, and some did. High level modulation transformers on low power transmitters were not as big as the one in Alfredo’s 500kw picture. This was a 1KW transmitter. Most transmitters of that size had the modulation transformer inside the transmitter cabinet.
They could say they were HIFI which meant they were a little better than the rest. Plus, playing the new hi-fi records of the time. It didn’t mean what it means today.
I beg to differ that the AM radios of the 50’s were not very good. Many weren’t but those older ones sounded better than most of the radios of the last 30 years. At least on AM. When FM became the leader for music, they quit making good AM tuners. They also limit the response on the transmitters. KGW had great sound on that old Raytheon transmitter they were using back in the 50’s. Before they moved in with tv, they did their broadcasting from the transmitter site and fed the output of the console directly into the transmitter in the next room. Not all stations did that though. When KVAN was at Jantzen Beach, the console was in the same room as the transmitter and they did not have good sound out of that old Collins board and transmitter. Same with KISN when they were from the transmitter. The quality of the sound was not good but teens did not care. It was about the music, not the quality. If you wanted quality, you would listen to KEX or KGW.
These were all done with tubed radios and broadcast equipment. Transistors had not maded their way to radios, yet.