So how many people are using subwoofers

I love my sub,it's addictive. If you crave bass athority get the sub, or better yet get two!

I would go for the amp change, but not necessarily with a higher powered version of the same solid state amp. Consider an OTL (the Joule sounds great with your speakers) or maybe Rogue monoblocks. My main speakers go down to 35hz and while I could always use more extension the bass output is more than satisfactory.

Since you specifically mentioned "bass weight" and not bass extension (the Sonatas go low enough) the sub may cause more problems. I've never heard a system with subs that sounded seamless, unless it was designed that way (Pipedreams and Airfoil come to mind, but they use multiple subs). Full range speakers are usually the desingers final statement and simply adding a sub is not that easy especially if the main speakers are ported.

I run dual Rel Stentors. You would be suprised on how much it will add to music. It will be fuller and smoother with better imaging and seperation.