If you like the DALI house sound, it's very possible that upgrading your speakers may scratch your upgrade itch. I have the Epicon 6's and love them. Even so, I use a pair of Rythmik 12" sealed subs to get the very low end. It makes a difference, and I fully agree with @soix regarding the improvements in soundstage and imaging. But echoing others, the point of diminishing returns is a deeply personal one.
+1 @socalml528 as well... just listening with eyes closed vs. open, light levels, etc., make a huge difference; by removing some senses from the listening experience, you improve the sense of hearing.