Subwoofer with center channel

You’re missing a ton!  IME the center speaker and subwoofer are the most important speakers by far in a multichannel system.  Properly dialed in a good sub (or better, multiple subs) not only adds bass but also ambiance, imaging, 3D soundstage, but it needs to be properly set up and dialed in to be seamless.  By all means break out that sub and put it to use!!!

I actually have three subs.  Two REL that run in two channel and home theatre.  A third larger SVS sub that is connected to the LFE channel for home theatre only.

I was thinking that there was a crossover setting for center channel in my Onkyo surround processor, but I can't find it.  Probably need to find the manual. 

I was thinking about adding another sub for just the center channel, but I don't know if it's worth the trouble. 

I don't think sub frequencies are sent to the center channel.