Rick Dau wrote: "For me, the litmus test is how well they sound on longwave...."
FWIW, I was just poking around on Bill's Lubec, Maine receiver, which Mark favors. While longwave is a bit noisy at midday, I was getting very good copy of two Canadian NDBs: YZX-266 Greenwood NS and ZST-397 St. John NB. I know you were checking SDRs in the southern U.S., but you might give Bill's receiver a try and see what you think.

Randy Stewart
Springfield MO