Correct, the DX-160 does not have an SO-239 (50-ohm) connector; it is a
high-z (500-600 ohm) connection already, and thus accepts a random wire
automatically "matched." If you had an antenna that terminated in
coax, you'd have to use a 9:1 matching tranformer "in reverse" to match
the impedance.

However, a 1:1 balun using ladder line or something similar might be
just the ticket...but the preselector idea sounds good too.
Bruce Jensen