The American Explorer series means to tell you that the great American musical subgenres are still living, breathing things, which, strictly speaking, is true. But that doesn't mean the subgenres are in their prime, and anybody who thinks these records are more important than most of the archival reissues now flooding the CD market is getting conned good. You want Johnnie Johnson at his best? Peruse the encyclopedic credits of Chuck Berry's The Chess Box, and feast your ears. And because the artists are somewhat less familiar, Chess' Bo Diddley box is even more educational, and almost as much fun. In country music, Jimmie Dale Gilmore himself would tell you that Rhino's George Jones and Lefty Frizzell honky-tonk compilations are more seminal and maybe more spiritual than ''After Awhile.'' Gospel collections are multiplying like sin, from pop hybrids like Al Green's One in a Million to the Abyssinian Baptist Gospel Choir's Shakin' the Rafters. Rock This Town: Rockabilly Hits, Vol. 1 sums up the randy teenage mood twixt '56 and '58; Vol. 2 attempts to prove that Elvises-come-lately, revivalists, and other diehards kept it juiced thereafter.