Thanks for the update. The DB-750X is listed as compatible with the AT-778UV cable, and topic 2711 has a DB-750 running Winlink over packet with it.
The VU meter dropping when the other radio keys up is normal: with squelch off, the idle level is receiver noise, and a carrier quiets it. So receive audio is reaching VARA.
One cheap test before writing the radio off: swap antennas between the Anytone and the Powerwerx. If the Powerwerx connects on the SignalStick, or the Anytone fails on the Comet, it’s the RF path rather than the radio.