KAB RF-1 subsonic filter

 I would have thought these very simple instructions would have come with the RF-1 from KAB.

They do. 

I thought KAB recommend install the RF-1 between the phono stage and preamp, so the RF-1 will be out of the chain when playback other sources.

https://www.kabusa.com/frameset.htm?/rf1.htm

The KAB RF-1 is easy to install. Place it in between the phono preamp and control amp or in a process loop.

Thank you for doing the necessary research, imhififan. I don’t doubt that you’re correct. What you write makes perfect sense in the case where one owns a separate phono and linestage, especially since it comes from KAB. I hope the OP reads your comment.

Yes, exactly. If you have a separate phono stage put it between the phono stage and preamp. The KAB then will have a consistent input and it will be less likely to be overdriven into distortion. You could also place it in a tape loop if you have one. 

The reason you want to do this is, whatever noise the filter makes will be attenuated along with the signal if the filter comes before the volume control. But after the volume control, the signal will be attenuated but not the noise! The signal to noise ratio will decrease along with the volume.