I'm going to fish out an article I've got somewhere by Jody Stecher on Spence. I seem to remember him saying (or someone quoted in the article) as saying that he did tune down a bit which accounted for that rumbling, out of tune sound and he also kept his 3rd string either sharp or flat in relation to the other strings.
#3 by David Lindley and #19 by Blue Murder (Carthy , Waterson, et al) on the Rolling Sea CD really floor me.

Frankie