Sean, I've been aware of the somewhat similar ACI filters, but I suggested the Harrison Labs items instead because they indicate a 12dB/octave slope rather than the 6dB/octave of the ACIs. This apparently means that they use a second-order filter with a series capacitor and a parallel inductor rather than only a series capacitor which forms a first-order 6dB/octave filter.
Have no personal experience with either one. You might take a look at the Harrison Labs site, especially page 2 of the catalog
where it shows how the varying amplifier input impedance affects filter frequency. I'd noted that Carl's 320 has a 20Kohm input impedance, so the result should be essentially the same as the 22Kohm designed spec of 70Hz.
You might also take a look at page 3 to read a complimentary comment about it from Absolute Sound. Note however that some audio enthusiasts would probably(with a touch of paranoia)reject these filters out of hand because of the "cheap" capacitors and inductors used. I recall an incident from about three years ago when Ian participated more frequently when someone indicated to him that he'd been disturbed on seeing the caps and inductors in the Axiom crossover and had the affrontery to suggest that "quality" replacements be made. Ian of course politely replied that the components had been thoroughly tested to provide optimum performance and that no replacements were necessary.
A brief comment on the STF-1/LFM-2 comparison being discussed here: have no idea about "lower distortion", but as I've indicated previously, the LFM-2 has a greater rated bass extension, possibly because the different port location allowed using a slightly longer tuning tube. This would result in a lower extension, with slightly less maximum output higher up, which some might consider to be a reason to slightly prefer the LFM-2.