It turned out that with 2 different SPL
meters there was a *measureable* difference. With the zip cord
sound was down 6 dB below 50hz compared to high-end cable.
I can reproduce this at will.

This indicates a serious problem in either the measurement procedure
or in your system somewhere. I'd suggest you spend some time looking
for what the problem really is, rather than drawing conclusions that
a cable change could cause it. I'd start by looking for something that
moved in the room, causing the bass standing waves at your SPL meter
to be different. Even not having your body in the same place for
both measurements might do it. Or, what about a really bad connection
in one of your cables, such as having only a couple of strands connected
to do all the work - maybe you really compared a 12ga cable against
a 28ga one :-)

Try measuring the voltage at the speaker terminals for each wire.
Does that show a severe difference? If not, the problem must be an
acoustic one. If so, look for a very bad connection or a very
unstable amplifier.

Bob