Home theatre subwoofer recommendations please?

I have an old (13yo) entry level at the time Onkyo home theatre system and the sub is starting to play up. I recently replaced the Onkyo receiver with a Denon x1400h. Its in our everyday lounge and gets used every night. Room is just an average sized maybe 6m x 3.5m max

Old sub is 8" and 75watt I think. Main use is movies, with a little bit of music.

Moneys tight, was hoping to save for one around $600, but after looking online think I may have to go closer to $1000?

Thinking 10" minimum or 12". I want to buy something half decent , in time (very distant future) I guess I may want to upgrade my other speakers as well

What are some good places to start? I see a KEF Kube10 300w (unported) sub under a grand, or there is a yamaha one for around 750.

Or do I keep an eye out for used on trademe? I see Velodyne impact 10 subs for sale sometimes. Although most used ones I have looked at online are probably at least 10 years old and they still want $400 for them.

Ideas please?