Carl, I did a double take when you mentioned receiving the Swedish station SAQ on 17.2 kHz with an HRO-500. But I see that the old (and rare) HRO-500 tunes down to 5 kHz. Congrats on also having the even rarer LF-10. preselector.
I bought the package around 1980 which was long before collectors got into the act.
I also worked at National and was the only one in the Service Dept who wanted to tackle the 500 and have been working on them ever since.
I tried copying SAQ a few years ago and heard nothing. From what I've been able to tell, hearing SAQ in the US is pretty much an East Coast thing... at best.
Ive heard reports from some places in the Midwest getting readable copy. Currently I use a pair of 750', 20 and 60 degree azimuth, Beverages and a HB signal combiner. Considering that one wavelength is 57209' its amazing it even works and I suspect it is just loosely coupling to Mother Earth 10' below.
I can also take the 230 kHz IF off the HRO-500 back panel and feed it into the TS-950SD to take advantage of the pair of 250 Hz crystal filters plus the digital audio filtering but most of the time I just use the 500/LF-10.
And I have heard it once (heard on the 500 first) a 1934 RCA designed WW2 USN RAK-7 which is amazingly quiet with a super sharp tunable audio filter. Too sharp and the alternator frequency drift will chop it....same as on the 950SD.
Carl