Regarding various receivers for LF. There are several varieties of Level
Meter available on the surplus market. Most of these make a good
sensitive RX on the LF bands. Some have adequate selectivity but this
can be enhanced with an outboard audio DSP unit for more flexibility,
another refinement could be a BPF for the required frequency 73 or 136
khz.between antenna and Meter.
Another approach would be to use the shack RX to tune across the Level
Meter IF output assuming it is a superhet, I think some older models
were straight receivers.
For those that are not sure what a Level Meter might be, it is an
accurately calibrated LF receiver although some have a range up to
several MHZ. .
One instrument that I have covers 200 hz - 620 khz and is in fact a
transceiver because it also has a tracking osc ganged to the tuning
device covering the same range on transmit. Output is several volts of
sine wave rf and useful for driving either a 73 or 136 khz amplifier.
73 de Mal/G3KEV