Actually, the thing you call a detector IS a mixer. You can probably
find some references for "H-mode mixer." It's good, to be sure, but
it's inaccurate to say that it's _perfectly_ linear. (I'd LOVE to find
a practical sampler which had zero distortion...though then I'd need
amplifiers with zero distortion, too...)

As someone else pointed out, any practical antenna you have for LF is
very unlikely to be a good match to 50 ohms, and is very likely to be
quite reactive so that by the time you add components to tune it, the
bandwidth will be pretty narrow. So if you have a tuned LF antenna,
which is quite usual, the response will be quite narrow, and why would
you care about a bandpass filter? I'd recommend a loop with a tuning
arrangement at its feedpoint (variable capacitance), and an appropriate
preamp to drive a feedline back to the receiver. With that, you won't
need any filter, just a transformer going into the mixer
(converter-detector-whatever). A while back, I did some work to modify
a design you can find at
http://www.cpinternet.com/~lyle/bal-pre/bal-pre.htm, so that the
control was done as a DC current , which also fed the power to the
preamp on the same line that signals come back on. It worked out well.
But check out other antenna options from Lyle's website
(http://www.cpinternet.com/~lyle/) or others devoted to LF, too.

Cheers,
Tom