Yes you should use at least one sub. Your speakers bass response depends a lot on placement and on the room itself. The placement requirements for your mains to provide good images and tonality are different from the requirements to provide adequate bass. For good bass, your speakers need to be close to room boundaries. For good image and tonality, they need to be away from your walls. Subs are the only way you can get both without compromise. And then there are these pesky room modes…