The Kenwoods I use have relay switched attenuators between 500 and 1500 KHz which is likely diode switched..... probably readily found on the schematics for other rigs. Below 500 they are very sensitive and a simple LPF for the BCB can be built in an hour. I dont have that problem with the various Beverages altho there are several 5-10KW daytime stations within 10 miles are so. Those either go silent or way down in power at dusk.
The K3 and K3S come built for 630
Carl