Even selctivity will not help all that much in the case of most
noise. Which by nature is broadbanded. When fightening
someforms of digitalnoise, a real narrow IF fitler can sometimes
help. But unless you are trying to dig CW out frm the man made
noise, super narrow filters are really much use. Much better to work
to reduce you local noise. A low noise antenna can really help. From
what I have read the Wellbrook loop,is great at reducing "point source"
noises. I have played with home made shielded/balanced loops, but
the results weren't worth the added trouble. I am currently playing
with a
sycnhronous noise blanker that "slices"/mutes the antenna output for an

adjustable number of mSec with a 60Hz line locked "switch". It almost
works.
Some of the Lowfers swear by this. The "swicth" has to go ahaead
of the radio's front end. I am strting to think that my 9:1 matching
transformer is "ringing".
Terry