I'd say my price range is about 100-200 dollars. I'll be using it mostly for music production and listening to my music. I want the subwoofer to be able to adapt to my environment as I move on in life but currently I use my speakers in our pretty large living room. But the question I really needed answered is my mixer problem. Will that mixer be able to adapt to any subwoofer?
That mixer has only one set of line outputs.
Your only solution is to split the outputs. However you can not connect the inputs.
So you need a sub that has R & L inputs. These subs have a built in buffer amp that will not make your main speakers mono.
If your sub only has one line input, you will need to purchase a zero sum buffer amp.
By the way, you don't want to go near that Onkyo sub with a barge pole. That will get you nowhere except waste money.