> I can hear the 125MHz tone very clearly if I tune into it directly ...
> USB power light is on and everything works well in pass-through mode (ie, I can
> receive stations above 25MHz just like I could before I connected up the Ham It Up)
That implies most of the board is in working order. Seems no low-frequency signal is getting to the mixer. It is possible the input low-pass filter is broken but...
> I've got a set of VHF/UHF rabbit ear (with UHF loop) antennae
MF/HF radio signal reception is highly dependent on the antenna. I can only receive my closest time station using two 60foot pieces of wire in a dipole configuration (one piece connected to the SMA center pin, other to the GND shell). Each piece is strung opposite the other along a wall.
Some people have been successul connecting the SMA connector center pin to a radiator or water pipes.
Please try such a larger antenna.