REL subwoofer

I currently have one REL R-305 sub with a ten inch driver in my system to supplement my Focal speakers and I feel that I would like a little more bass from my system. I have considered purchasing another sub identical to mine but space constraints may pose a problem. Is a better solution to purchase one 15 inch sub like model up from mine in the REL line-up or would two smaller ones be better?
How do you define a little more bass? What bass region are we talking about? Have you tried repositioning you main speakers and sub? Experiment with placement before you spend more money. BTW, how big is your room?
I have a R305 and really like it, in terms of sound quality it's the best sounding sub I've ever owned regardless of price. Unfortunately the R305 isn't well suited for large rooms or high volumes. It's great for music but if you are looking for high SPLs or real LF performance you'll need a B1 or JL F112 or 113.