[Elecraft] K3: Sub receiver goes silent at narrow bandwidth

[Chuck Teague via Elecraft]

My first post on this didn't make it to the list due to clerical error on my part, so here is attempt two.
The sub-receiver in my K3 is equipped with a 2.7 5 pole filter in position 1, and a 400Hz 8 pole in position 5.  All works well until I narrow the bandwidth to 400 Hz, then the sub-receiver goes quiet (the main continues to work normally.)  I used the K3 utility to set filter 5 bandwidth both narrower and wider, and wherever it is configured is where the receiver goes quiet.
I can't find anything in the archives about this particular problem, but thought I would seek the group collective wisdom before tearing into the radio. Thanks,
Chuck Teague, NN7U