I suspect the SRF-A100 switches in a synchronous detector when in stereo mode, even if the signal is in mono.
I also have an SRF-A1, an SRF-AX15 (a Japanese model with 76-108 MHz FM coverage), and an SRF-42 (AMAX model). The A100 and A1 were bought when I was in Houston. At the time, KKBQ(AM), KRBE(AM), and KYST(AM) were in stereo (790, 1070, 920 respectively). KYOK (1590) claimed to be in stereo but really wasn't. In the 1990s in Kansas City, I still got some use for those stereo models: WDAF (610), WHB (710) until 1993, and KFEZ (1190). When I arrived in San Francisco in 1999, KFRC (610) was still in stereo. Not sure if KABL (960) was in stereo that year, but it was later and was the last AM stereo station in the full market. KVON (1440) in Napa is in stereo but it's pretty much limited to Napa and Sonoma counties.
I also have an SRF-A1, an SRF-AX15 (a Japanese model with 76-108 MHz FM coverage), and an SRF-42 (AMAX model). The A100 and A1 were bought when I was in Houston. At the time, KKBQ(AM), KRBE(AM), and KYST(AM) were in stereo (790, 1070, 920 respectively). KYOK (1590) claimed to be in stereo but really wasn't. In the 1990s in Kansas City, I still got some use for those stereo models: WDAF (610), WHB (710) until 1993, and KFEZ (1190). When I arrived in San Francisco in 1999, KFRC (610) was still in stereo. Not sure if KABL (960) was in stereo that year, but it was later and was the last AM stereo station in the full market. KVON (1440) in Napa is in stereo but it's pretty much limited to Napa and Sonoma counties.