To make a long story short, I purchased a DE-31 loop for portability
and it came with a 3 conductor mini-jack to "two clips"
(antenna/ground) adapter. The antenna works fine with the minijack /
but not the clips. I tried the clips with my Edek Torus Tuner... the
clips don't seem to work with that either. The clips are making
electrical contact through, though. I tried the clips with two radios
(one radio doesn't have an antenna jack - hence the issue).

Important details:

The DE-31 has a 3 conductor mini-jack and the adapter ends with TWO
clips, one for antenna and one for ground. The radios don't have
"ground" connectors per-se, so I'm not sure the best place to connect
the ground wire and if that's causing an issue.

When I connect either antenna via the minijack, I can tune the antenna
just fine (I hear the "click" around the proper spot and then tune
slowly) and it makes a marked improvement.

When I connect the Torus Tuner via the clip adapter, I can hear the
"click" and fine tune, but it makes very little difference (on the same
frequencies I tested with the minijack). Remember that the Torus Tuner
has a 2 conductor plug and the adapter is 3 conductor - I don't know if
that makes a difference.

When I connect the DE-31 via the clip adapter, I can't hear the "click"
at all. The DE-31 has a pretty long minijacked wire to connect to the
loop. When I use the clip adapter, removing the wire from the loop,
gives the same reception as having the loop attached.

That is, it seems that on the DE-31 at least, when it's connected using
the clip adaptor, it's using the wire as a 'longwire' antenna, but it's
not interacting with the loop at the end of the longwire correctly (and
reception using the wire varies by frequency). Is this an impedence
mismatch? Could this have to do with not connecting the grounding wire?
Any other possibilities How can this be tested (I do have a
multimeter)/ corrected?

This thread has technical data regarding the insides of the DE-31 if it
can be of any help.
I have not moded the unit:
http://www.drmrx.org/forum/showthread.php?t=1210

Thanks for any insights!