prog-metal????? - prog-soul maybe 
Certain Rare Earth tracks do warrant a look at by prog fans I think. The 23 minute live version of "Get Ready" on the In Concert LP is splendid. Lots of interesting solos on bass and keyboards to boot and one of the most underated double live LPs of the early 70s.
There firts few LPs are of interest to Rock fans - Get Ready, In Concert, Ma (another side longer track) and especially the Ecology LP, with its amazing version of "Elenor Rigby". Rare Earth despite not being symphonic prog or anything - are still a great early 70s rock act full of intelligent playing, extended solos and brilliant singing. Bringing a prog-like nature to the Rock/Soul canon. Their version of "Smiling Faces Somtimes" is as good as The Temptations own version.
Overall a great band RARE EARTH