It's all about matching the subs capabilities and when one drops out and leaves just one sub picking up the slack, there's a corresponding drop in volume. The more diverse the subwoofer capabilities, the more difficult it will be, trying to integrate the bass reproduction system into the listening venue's acoustics.
I couldn't find any specs on the Onkyo, SKW-580 subwoofer so you'll have to go it further with somebody else.
Basically, FWIW, at the lower end of the price spectrum, in the case of stand alone subwoofers, my opinion, all one is doing is augmenting the mid-bass of their main/center channel speaker system as opposed to actually helping the lower frequencies; <30Hz. In this case, my opinion, one is better served upgrading their main/center/channel speaker system by stepping up and acquiring a better grade subwoofer. Personally, in polite terms, I don't have an affinity for these types of subwoofers as I personally think one is throwing their money away (wishing-and-a-hoping) and could get better bang for their buck stepping up to the next grade in subwoofers; the $200.00 to $350.00 range.