is a cheap subwoofer always better than using none at all?
Not sure what the low end of the Sony’s is, but I would be willing to bet that even a little 8” will bolster the low end for music at least. High volume levels or bass intensive music might stress it some though.
Depending on the size of the room, it probably won’t do a lot for HT as it won’t have much extension, and certainly won’t have much useable output below about 30hz, where good movie bass is.
I would definitely try it out, but definitely keep an eye on a future upgrade.
Oh to your last question, like usual, it depends... lol