Reference strada one or two subs?

I currently have the reference strada side speakers on floor stands and a rel t1 subwoofer. All driven by a rega brio-r amp. Love the sound just maybe lacking a little bit in the low end. Wondering what adding another sub might get me. Any one have any thoughts?? I'm too poor to go for the reference 3.5s but hoping to get some good advice on how to improve the setup. Many thanks!