Originally bought it for the Arc gen 1 and the Sub 4 made a massive difference to bass and allowed the arc to improve in the mids and highs too. However, then I upgraded to the ultra and 80% of the time, I’m pretty sure that the ultra is handling the bass and doing it very well. The other 20% for deep, sustained rumbles, the Sub 4 certainly delivers. Buttt, quite an expensive addition for something that only performs and is necessary for 20% of the time (at most). So, if you have an arc, go for a sub (any gen). If you have an ultra, upgrade the rears before the sub. Anddd, if you already have a sub, then don’t bother getting the Sub 4, unless you need the matte finish (which in the white is barely noticeable from a distance but is probably a fair reason to upgrade if you want the black and have it in view). All that being said, great and very happy with it.