In terms of what can make a massive difference for not much outlay, I found that standing my Kef iQ90s and Kube2 sub on solid marble bases (inch thick) made the biggest improvement to sound and clarity, as they stop any vibrations being lost via the flooring. What speakers are you running btw? And are any of them bi-wired? There's a subtle but marked difference in sound quality if you can do that (my fronts are bi-wired).
As a more general idea for others, unless you have plenty of Atmos-soundtracked sources then it may not be worth the outlay going for any kind of Atmos setup, and certainly ceiling mounted is preferable to upfiring if you possibly can.