I'd hardly call it heresy, John. In my search for really good op amps
to use up to 50 and 100MHz (very low distortion and low noise), I've
come across some that would be really outstanding up to a couple
hundred kHz.

In fact, since you can get 24bit ADCs that cover up to that range
(e.g., AD7760, AD7762) with very good linearity and low noise, you
could make the whole LF receiver with just the tuned antenna, the
preamp out at the antenna, _maybe_ a bit of gain, and the ADC feeding
into a PC. Then the "quadrature mixing" would all be done digitally,
with much better accuracy than you'll get with an analog mixer. Yeah,
yeah, you have to write some software to get it to work...but a modern
PC should have no trouble keeping up with doing all the signal
processing. There may even be sound cards out there with response out
to 100kHz--that's an area I don't keep up with.

Cheers,
Tom