Each filter attenuate at least 1-2dB each on all frequencies. 3dB means you loose half your signal as if your antenna got cut in half. Depending of what scanner you have, what antenna you use, how high signal levels you have, what frequency you are monitoring, will affect the result if you gain those dB back by filtering out an interfering signal.
I have three 60kW FM broadcast transmitters 5 miles from me and there's four others at 20kW and six HD tv transmitters at 470MHz and higher of unknown power in that same tower. Only the 68-88MHz band are affected in my scanners as that pre-filter in most scanners covers 50-108MHz, and the airband and other frequencies are not noticable affected by the transmitters. I use several $30-$40 LNA's in my setup that can handle +20dBm signals.
/Ubbe
I measure my fiolters using the appropriate equipment. Generally, if I can measure 2 dB away (say 10 MHz) from the notch frequency, I throw it away (and have done that regularly as expense).
I also use multiple filters and LNA's (commercial ones over $100). I have 2 FM broadcasters near me (one at 0.8 mi and the other at 3 miles). Even without the LNA, the filters would be needed as I listen to a wide range of frequencies and find problems if there are any. Even though you cannot hear the FM broadcasters when listening in the 150 and 160 MHz area, the receivers are de-sensed without the filters or suffer from intermod or harmonics (note: these are good receivers, including an ICOM R9500). And yes, there are still some of us who have VHF high band pagers nearby. Filtering those without affecting nearby frequencies of interest is a challenge.
My usual configuration is two FM broadcast notch filters, then depending on what I am doing, ones for 152 MHz paging, 2 meters (a local amateur repeater is on the tower at 0.8 mi), and a local VHF-Hi FD repeater. If I am focusing on VHF aero, I use a band-pass filter for that (made by Rockwell). These are in front of a good LNA (>$100). It took time, experimentation, and of course money, to find a satisfactory combination that is local dependent. Some make the mistake of not wanting to notch something they want to hear--often that is a local that is going to get through anyway and notching it will help hear things away from that frequency that may be of interest. And of course, I use multiple antennas, with different notches.
(Adding the caveat that antenna height should always be a first goal.)
I consider yearly visiting the LNA-notch configuration part of what one needs to do if one is serious about optimizing the set-up.
