Do I need a separate subwoofer with a soundbar?

It depends on how much low-end you want. Reviewers consistently note that all-in-one bars with built-in or no subwoofer (like the Sony HT-S100F or Bose TV Speaker) are a big upgrade over TV speakers but run out of deep bass. Models bundled with a wireless subwoofer, such as the Polk Signa S2 or Sony BRAVIA Theater Bar 5, deliver noticeably more impact for movies and music according to independent testing and verified-purchase reviews.