I use an audio spectrum display - Spectrogram - to fine tune a barely there weak signal. Set tuning rate to 1 Hz - the APF peak is less than 10 Hz. Set the frequency cursor in Spectrogram to your pitch frequency. Set the DSP bandwidth wide so the NR works effectively ~ 700 Hz. Use NR mode ~m5-3 so the APF has some unprocessed signal to work on. It takes practice but works.
important is to set fine tuning (1Hz) when using the APF as the only precise
tune on spot will effectively pulling out this weak sig from the noise. I do
not understand why the K3/K3S never got as same feature as KX3 > switching
to fine tuning automatically when APF is ON...
Also try to reduce RF gain (do not keep it at max) in order to reduce noise.