IO540
I've seen some cavity filters that were very impressive, but hardly suitable to aviation use.
Active filter - in my way of looking at it, you're talking a "front end" rather than a filter. Yes, that could be done - might even cost £50, plus a few hundreds of £k to get it certificated. A bit like the DSP on my HF kit here - brilliantly effective.
There was a better answer ... but the Government wouldn't have made much money out of that. And we wouldn't have had to pay for the solution.