I think this is getting a little carried away :lol:
We're only discussing interference on a cheap MW radio, I still don't think it's been confirmed WHERE the interference is entering? - mains, RF, or perhaps directly in the IF?.
There seems little point getting complicated about filters, when we don't even know where (or what) it might be used for.
The main complain (as I see it?) appears to be that an expensive radio is better than a cheap radio - personally I don't find that surprising?. I thing it's probably unlikely to be down to adding some magical filter, just that the overall design is going to be better right through.
I can't remember the last time I ever used an AM radio?, although I still have one of the best ever made, a Hacker RP35. For those of you who have never heard of Hacker, they made the BEST portable radios in the world! - bar none!. Unfortunately, people ceased to be prepared to pay for quality, and Hacker went bust :cry:
I bought mine second hand in 1971, it was the loudest portable radio you ever did hear :lol: it runs off 18V - two PP9's in series, and has a large elliptical speaker.