The subwoofer output on the amp is designed for powered subwoofers, and every outdoor subwoofer I’ve seen is passive, requiring connection with speaker cables, same as your rock speaker. Theoretically, you could connect a sub to the same amp directly, if your speaker pair and the sub are 8 ohm, however, I would get an impediance matching speaker switch to be on the safe side. If you find that you’re not getting ‘enough’. You may want to get a separate amp for the subwoofer, like the product below.
https://www.outdoorspeakerdepot.com/amplifiers/subwoofer/500w-mono-channel-class-d-digital-subwoofer-amplifier-smp300.html
I should add that many people that bass frequencies don’t travel well in an outdoor environment and that an outdoor subwoofer is a waste of money. I can’t back that up from personal experience, but the fact that are relatively few outdoor subs available compared to outdoor speakers on the market, it does seem like it’s not a very popular option.