Here is the FlightAware Pro Stick Plus + FlightAware external antenna + Uputronics 1090Mhz SAW filtered preamp. You can see what a great job the preamp does of removing comms and mobile signals and boosting ADS-B.
Click the image once to open, and a second time to zoom to normal size, then scroll left and right to view the comparisons at different frequencies.
You can see that the Pro Stick Plus does a good job of filtering but still picks up a fair bit of unwanted stuff, and the filtered preamp nicely takes care of that.
I’m in the UK. You can see digital TV, comms and GSM signals present without the preamp.