The answer to “Do I need a sub?” is always yes. Even if you already have one, or two, or three, or more 😀 Multiple subwoofers is the path to high end bass and high end audio in general. Yes, if you high pass your mains they will play louder and clearer because it limits cone excursion on the lowest notes and hands that duty to the subwoofer which is designed specifically for that task. That’s what I do in my system and it sounds great. I’ve got two subs and my end goal is at least four. Not for loudness (although that’s fun sometimes) but for bass even-ness, extension and clarity. When you get a sub, invest in a MiniDSP module or something with DSP capabilities. It lets you precisely control the crossover and crossover slope, apply notch filters to help minimize room resonances, and do other things like apply Linkwitz transforms which can extend the bass lower as long as you have the amplifier watts and available cone excursion (for sealed subwoofers anyway, not sure if that works with ported). DSP and subwoofers should be married to each other in high end audio. Best of luck to you.
