This Boston quartet first united 14 years ago as a bizarre venture into avant-garde country music. Now, armed with their polished sixth LP, that rootsy experimentation must feel like a foggy memory. Free Yourself Up , like 2016’s Side Pony, is a subtly sophisticated soul-pop romp, led by the jazz-trained belting of frontwoman Rachael Price and the low-end thump of double-bassist Bridget Kearney. It’s a malleable style they can gracefully bend into adult-contemporary jazz (“Shame, Shame, Shame”), twangy psych-rock (“Dude”) and funky blues (“Red Light Kisses,” highlighted by Mike “McDuck” Olson’s vintage clavinet and organ)—mostly because they’re such strong players and arrangers. From the overdriven guitars of “You Are Free” to the fidgety 5/4 groove of “Hang On” to the purring vintage synths of “Baby, Don’t Leave Me Alone With My Thoughts,” these tracks are loaded with subtle surprises that transcend the “retro-soul” tag they’re often saddled with. But they’re also smart enough not to bury the lede: Price’s smooth tone is the big-ticket attraction, and each track is loaded with hooks that showcase her raw talent. She revs up into a bluesy purr on “Good Kisser” and reaches back to her jazz roots on “Musta Been Something,” a deft slow jam full of dramatic swoops and trills. While Lake Street Dive clearly course-corrected from their experimental early days, they never abandoned their instinct to blur boundaries.