I should mention from the outset that I got to know globe-trotting old-time banjoist Abigail Washburn pretty damn well — and to like her a lot — while doing hours of interviews with her for a book I published last year. But that’s also what took me knee-deep into pondering what sets her apart from the whole wide rootsy world of confessional, autobiographical songwriters. One answer I came up with is that what she does goes well beyond means-to-an-end co-writing, all the way to an open-hearted embrace of collaborating with distinct musical voices. Hence Abigail Washburn and the Sparrow Quartet, her viscerally adventurous meeting of virtuosos of a few years back, and last year’s City of Refuge, with its burnished hooks and horn parts born of a partnership with imaginative multi-instrumentalist Kai Welch. Phoebe Hunt is another kindred spirit joining them for this show, a former member of Austin’s Belleville Outfit with loads of fresh ideas about how to put a fiddle to use.