One thing about many of the old vacuum tube and most solid state receivers is they have dynamic RF amplifier front ends. This was a design trade-off made back in the earlier days of radio to quickly get multiple band coverage without the need for plug-in coils for each band as with the National HRO receiver line for example. Most receivers rely solely on IF filtering for selectivity. This design trade-off resulted in very poor RF amplifier selectivity with hardly any rejection to strong broadcast station interference or other interference down lower starting at DC on up. This can often lead to many receivers being unusable at certain locations and even intermod products getting through on the higher bands. I recently purchased the premium Kiwa Electronics filter attached below and I am currently using it on my NC-303 receiver. I need to modify the receiver's input to a 50-ohm termination though to get the best performance. I'm thinking of running coax from the SO-239 connector on the back of the receiver to a 51-ohm resistor to ground to the 100pF coupling cap then to the 1M grid-leak resistor.