Just get five speakers. No new sub is required.
Subs are pretty universal and can easily be mixed and matched with virtually any brand of speakers.
My suggestion, it music has any place in your system, is to search for the two front right and left speakers. Bring MUSIC to audition them, not movies.
When you find a pair that sounds good on music, go for them, and the matching center and surrounds from the same manufacturer/line.
Any speakers can sound impressive with the boom/tizz of movie soundtracks, particularly action movies with a lot of action.
It takes a speaker with finesse to do music well.
If a speaker can do music well, it can do movies well. The opposite is not true.
Once the speakers are installed in your system, blending your current sub will not be a problem.