I recently switched speakers within the same line, from a large'ish (115 lbs + stands) floor-standing, bass reflex (i.e., ported) model with multiple (4) medium sized (7.5 inch) bass drivers per box, to a slightly lower-priced, heavy (105 lbs + stands) stand-mounted model in a sealed box (i.e., acoustic suspension) with dual 9-inch bass drivers and mid/high frequency drivers that are very similar to those in the floor-standing speakers. I added a second high quality sub (two total) and run the two subs in stereo (i.e., signal from one channel to each sub, not a summed signal).
IMO, the gains include better defined bass above 40 Hz
and better bass roll-off above 40 Hz (from the main speakers), which also improved the clarity of the upper frequencies. The two subs provide better defined and harder hitting low bass below 40 Hz as well as much better control of the bass by adjusting subwoofer placement, cross-over frequency, and volume. In addition, the two subs each have a remote volume control so I can easily adjust the bass for a given recording without affecting the range above 40 Hz. IMO, it was a big improvement for a cost increase of around 25% of my total speaker cost, calculated based on retail pricing.