If you're using a MW filter and you're still getting good MW signals, then either the filter doesn't have a high rejection, or/and the signal on 1450 in phenomenally strong. A MW transmitter at two miles... you could power an LED on that from your antenna. Worth a try!
Also 1450 is getting towards the end of MW so the attenuation skirt may not be very steep.
Worth noting the RSP1a has a MW built in filter. You could add your filter to that also, if need be.
Edit: That RTL filter claims 60db of suppression, which is pretty respectable, though, again, if you still have very strong signals, the above still applies.
Also... that filter may be reaching saturation and not actually blocking that signal. It could even radiate from the input to the output, inside the housing and miss the filtering completely, due the signal strength. Make sure that your radio is well-grounded. you could be getting that very strong signal over the shield too. Remember that you're dealing with electricity.
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