I want to improve my current setup. So I have a bunch of questions.

This is my current setup:
Sony STRDE-635
(2) JBL N26
JBL N-Center

Its not really the best but it's better than nothing I guess. I would like to complete the 5.1 system at least.

My first question is what would be a good sub for ~$300. Look around I think that the Hsu STF1 would be a good choice. Is there any others I should look for?

I don't think I can afford to purchase a complete set of speakers. So I was thinking that I could use my current N26s are rear speakers and get a better set of fronts. Or should I do it the other way and get something better for rears?

If so what should I get? I want them to be mountable to the ceiling.

Thanks in advance