I just did a quick test, adding a low cost subwoofer to my old Pierre-Etienne Leon M1 speakers, and it made quite a difference.

If I could find a matching center, I would consider the option of keeping them and add a sub and surrounds.

I have no idea how I could determine what center could match.

Yamaha HTR-6190, MA RS6 & RS-LCR, DSP-3100, PEL M1, Samsung LN52A750, SA 8300HD, Home Theater PC