What is this, the Terence Hallinan Memorial? (Speaking of punchy....)Attaboy, @Weiserguy ! And while we're at it, it's the Fog City Diner, the Nimitz Freeway and Long's Drugs, dammit!
Get me Van Amburg and Herb Caen! We'll get this straightened out.
When the simulcast first started...in 2008...which means it's been going for more than 17 years...there was a lot of discussion on Usenet's ba.broadcast about the processing on the FM signal. Namely, there didn't seem to be much of it, about on the level of KQED-FM. There was also some consternation about the station's deciding to remain in stereo, but that's a separate topic. Over time, the processing has been beefed up somewhat on the FM. On the AM, you may also be hearing the effects of aggressive AGC in the receiver. Most DSP-based radios seem to have aggressive gain on AM, which punches up the dynamic range compression. Tecsun radios seem to be especially prone to this, so much so that a combination of station processing and receiver AGC causes the audio to pump in and out on some stations. A few years, back when I was visiting Sacramento regularly for work, I noticed this on the station at 890.KCBS does sound good for an AM. And I'm just enough of the right kind of geek to admit that their top-of-the-hour sounds a lot punchier on the AM than it does the FM. More aggressive compression.
The AM audio frequency response is limited to 10 kHz, so that also means processing doesn't have to take into account frequencies above that limit. That might make the AM version of the audio sound louder, at the cost of losing a little sizzle, to use a technical term.