I had gotten that info from Roger's site.
Maybe I'm misinterpreting...
http://www.roger-russell.com/tuners.htm#mr80
** You're correct the owner's manual doesn't specify stereo or mono - so probably mono. **
MR 80 FM tuner.
ELECTRICAL: Sensitivity 9.3dBF or 1.6uV for 30dB of quieting. Stereo 50dB quieting sensitivity 14.7dBF or 2.5uV for 3% noise and distortion. Response 20-15kHz (+1 -1dB). Distortion 0.2% in stereo. Capture ratio 1.5. Image rejection 90dB. S/N 75dB. Alt channel selectivity 90dB (narrow) and 110dB (super narrow). Adjacent channel selectivity 8dB (narrow) and 60dB (super narrow). Separation 50dB@1kHz. Spurious rejection 110dB. SCA rejection 60dB.