At first I was reluctant to answer this question, as I believe in order to do this properly, will cost more than the value of said sub-woofer.
However as nobody else has commented, I will give you an answer on how you can do this.
There are two problems we need to overcome:
The mixer has no further outputs to connect the subwoofer.
The subwoofer has ...
There are a few possible reasons. If you see just two identical mics it's for coverage and diversity -- to get a good signal no matter where the speaker faces, to shape the pickup pattern.
If you see multiple different mics it's usually because the venue didn't provide a 'gang box', a splitter that can supply feeds to many different sources, or one or more ...
A few quick notes
At stage volumes, you really really want to be using a SSD Drive. You will have issues with normal platter style HDD's at stage volumes due to vibration
Even with a SSD, you're going to want to sit the laptop on some vibration damping type material. Grab some mopads (designed for acoustically decoupling speakers) and sit it on that, ...
It's not ideal, but if you don't use the headphone output on the mixer, just run a stereo 1/4" to rca y-splitter cable to the rca inputs of the subwoofer. Otherwise, I'd have to agree that it would be more trouble/expensive than it is worth.
I wouldn't run stereo in that setup at all.
The closest approximation you would achieve would have only those in the centre of each block being able to hear the image...
You would also only be able to give any sense of directionality to those at the sides. Those sitting top & bottom would have stereo, but it would be reversed.
I'd just run mono.
There are many different ways of recording 360 degree audio, binaural is probably the most common for for example VR/AR (usually 'headlocked', meaning the sounds stay in the same place relative to head movement), ambisonics is another way to do it. In each category there are many different possible options of recording it with each their own specific gear. I ...
PreSonus has come to the rescue! Check out:
PreSonus MicroStation BT
at your fav search engine. It allows you to connect your mixer to active speakers (studio or pa), as well as your hifi rca input active subwoofer for 2.1 outputs. It even throws bluetooth connection into the mix as well as an extra headphone connection and a volume control.
And no, I am not ...
@Edwin van Mierlo's answer is correct. But if you're in an environment with not too much RF noise, instead of the expensive converter, you can use an XLR to RCA cable from the MIX output on each speaker to the sub.
Ultimately, each group doesn't want to be responsible if someone else's audio doesn't work, so they get their own. It's generally simply because they don't want to share/rely on each other and the venue or speaker doesn't force them (or can't provide for them) to play together.
I think it may just be that your Dell is not up to the task - that's a pretty old machine (2005!). It could be simply failing in one or multiple ways. You could format it, re-install the basics of what you need and try again, but chances are it's not up to the job.
Personally, I would never trust a laptop this old in a live situation if there was anything ...