“Progressive rock” is a loaded term, conjuring notions of epic keyboard solos by men in capes, lyrics concerning warlords or gurus (or warlord-guru Ayn Rand), and drum kits wide as city blocks. It’s worth remembering, when approaching the third album from Boston sextet Bent Knee, that some of the most genuinely progressive items in the canon were fashioned by pop artists flouting convention: a lineage extending from “Sgt. Pepper’s Lonely Hearts Club Band” and “Pet Sounds” to “Homogenic,” “OK Computer,” and arguably even “Lemonade.” Exalted company, but it suits the gumption displayed on “Say So,” an album more concerned with songcraft than showboating, and aimed less at escapism than at self-examination, however elliptical. The playing is polished but rarely extravagant, the writing a mix of singer-songwriter contemplation, baroque construction, heavy-metal swagger, and virtuoso assurance. Lead vocalist and keyboardist Courtney Swain’s stagey delivery exerts a singular magnetism; she’s an idiosyncratic siren, her call amplified by bandmate Vince Welch’s consciousness-altering live electronics and sumptuous production, and emboldened by the group’s cheeky versatility.