I didn't notice whether or not the OP said that he borrowed a known good
headset, since that's the one thing that is common to both rigs.

Several headsets were tried with same results. Both these headsets
worked fine in wife's C-172 (ATC reports crystal clear)
One of the radios has a new store bought antenna on the belly with new
BNC connectors that came on a "factory built" length of coax to the
radio which came with an antenna "pigtail" already on the tray. This
complete replacement setup seems to xmit no better than the "cheesey"
antenna and antique radio it replaced and is reportedly no different
than the 2nd radio/antenna setup.

Jim