For years, bluegrass performers have sought the elusive formula that would combine their music with mainstream country without diluting the strengths of either. Bluegrass veteran Ricky Skaggs discovered that holy grail last year with his breakthrough album, "Waitin' for the Sun to Shine."

Last night at Wolf Trap, Skaggs and his eight-man band gave a clinic on how it's done. They did it by keeping the fundamentals simple and sharply focused so the lead vocals and solos could shine. The rhythm section applied a firm but understated thump behind Skaggs' vocals of old-fashioned heartbreak and affection.

Skaggs sang "I Don't Care" as a plaintive mountain lament but was pushed along by Jesse Chambers' insistent bass. "Highway 40 Blues" was carried along by a familiar on-the-road momentum but was given a back-roads flavor by Skaggs' mandolin and Bobby Hicks' banjo.

Hicks and Lou Reid played a swooning fiddle duet under Skaggs' resonant confession on "Cryin' My Heart Out Over You." When the band played an old Bill Monroe tune, Northern Virginia's Bruce Bouton echoed the fiddle part on pedal steel guitar.

In the opening set, the Seldom Scene gave a delightful demonstration of the type of progressive bluegrass Skaggs used to play. The all-accoustic quintet picked out clean, sharply focused lines that never got cluttered, even at brisk tempos. Their three- and four-part harmonies were not only expertly balanced but supplied an understated drama to heartbreak songs like "Wait a Minute."

Mike Aulridge's virtuoso dobro solos supplied a lonely moan to many songs, especially J.J. Cale's "After Midnight." Surprise guest Jonathan Edwards joined the Seldom Scene to sing "It Keeps My Feet on the Ground."