If you had a bow arm like Brittany Haas, you’d probably set aside your training in marine biology for the life of a semi-itinerant fiddler, too. Thanks in part to her work with prominent contempo string band Crooked Still, Ms. Haas is in the forefront of young players who lay an omnivorous fascination with nearly every sort of music on a strong groundwork of traditional styles — and Nashville’s lucky to have her, even if she’s on the road more often than she’s home. This year, she’s hitting it hard with fellow fiddler Lauren Rioux, and the pair worked up an exquisite show that’s heavy on the violin, but features banjo, guitar and sundry other strings worked in as well. Cool arrangements of old fiddle tunes? Check. Nifty originals that sound by turn old-timey, classical and just plain unclassifiable? Check. A winsome vocal or two? Check. And that’s just for starters.