Does using a powered subwoofer make better sound?

In short: yes, a powered sub -- if it's the right one -- could definitely enhance the performance of your system. There have been a lot of posts on this subject over the past 3-4 months. I contributed a long piece which summarized the results of Widescreen Review magazine's tests of some 50 subs -- you can look for the post in the Audiogon archives, using the phrases "Widescreen Review" and "subwoofers".

Also, look for posts from other regulars on this forum. My personal experience with powered subs relates mainly to my Vandersteen 2Wq subs, which I think are outstanding, but other members of the forum have made excellent posts about other brands/models that will be informative for you.

Based on your situation, I believe a REL subwoofer would be a good fit (specifically, I'd look for a subwoofer with speed). The model that you will need would depend upon your room dimension. There are some trade-offs when using a subwoofer but it is my personal opinion the benefits prevail, especially in dynamics and bass when a speaker needs a little help in those areas. You will need to do a little work to position the subwoofer in the right place and to dial-in the crossover.

There are many threads that you can refer to for more opinions concerning your question. Good luck!

I second the REL suggestion. I haven't heard a system yet that doesn't benefit from a REL.