The K3 sensitivity falls off severely below 160 meters. But if you have the IF option (KXV3) which provides a receive antenna port it does not have the filtering, and there is plenty of sensitivity over the whole band (0.5-1.7 MHz). The KXV3 is very useful for 160 meter use as it allows use of a Beverage. You should be able to make your signal level/readability tests without a seoarate AM filter. I just checked on my K3 and sensitivity falls off very fast below 1.6 MHz.
Rick K2XT _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK