Yes, I have the external highpass filter sold by Kiwa. You can find
information about it he http://www.kiwa.com/bcb.html.

Since MW DX-ing doesn't interest me very much, and since it's a pain to
be tuning a preselector all the time, it's nice to have a filter like
this that will heavily attenuate *everything* below 2500 khz.

I.C.E. sells a comparable highpass filter (model 402), and I believe
Par Electronics does as well.

Steve