...is highly dependent on getting the filter settings right (whatever they're actually doing)
The receiver chip are designed for satellite reception that has 7Mhz or wider signals and not really doing well at more narrow signals that are received with scanners. The filter has in its Off setting a wide window like 5Mhz below and abow the frequency you are monitoring and the receiver will be affected negativly by any strong signal within that +/-5Mhz window.
What filter settings do are that it's shifting that window to either side of the frequency in different amounts to try and block out a frequency that are interfering with your reception. The Normal setting are a high pass filter setting that moves the filter uppwards in frequency and let all frequencies 10MHz above your monitored frequency to pass thru and hopefully there will be no strong signals there. It will then also block all frequencies that are lower in frequency and hopefully the interfering signal are below your monitored frequency.
If the interfering signal are above your monitored frequency you would insted choose the Invert filter setting.
If you have several interfering signals that might be both above and below your frequency then a Wide setting might help as those only shift the filter half ways, like 7,5Mhz in one direction and 2,5Mhz in the other direction.
There are different mixing products, that are unwanted, within the receiver chip and choosing IFX to a frequency will change the frequency of those mixing products and move a received signal that actually are transmitted at another frequency to some other place in the spectrum that hopefully will not be heard or interfere with reception. It's sometimes needed to use both IFX and filter settings to to try and reduce any negative effects from nearby strong transmitters and sometimes that doesn't help and you have to use the attenuator setting.
There's also an automatic gain controller that reduces the gain in the receiver to keep it from overloading from too strong signals. That AGC works independent of the filters and directly at the front end of the receiver and are more than +/-7MHz wide, that suits broadbanded satellite signals but not so much 25KHz narrow 2-way radio signals, that makes a SDS scanner loose sensitivity if it sees a strong signal withing that frequency range.
/Ubbe