SOTABEAMS has just announced a range of ready-built audio filter modules.
Each module has two different filters which can be used to improve the selectivity of most radios.
The narrow CW filter has a tuning indicator for easier use.
The modules have been designed to be simple to use, requiring just power and audio connections.
Naturally, given SOTABEAMS portable pedigree, they have a low current requirement too (30mA).
More information - http://www.sotabeams.co.uk/digital-audio-filter-modules/