''Bluegrass supergroup'' may sound like an oxymoron, but you've earned the title when you boast five singer-songwriters whose résumés include stints with Earl Scruggs, Alison Krauss, and Ricky Skaggs. On Marbletown (out June 7), Blue Highway apply their hot licks and high lonesome vocals to a stack of rueful tunes. Highlight: Guitarist Tim Stafford's waltz ''Quarter Moon'' sounds like an instant classic. Why are these guys super? They can make your heart soar even as it's breaking.