What are the limits of room correction/ treatments ?

I would stay away from room correction devices and focus more of your effort on the room it self including placements and isolation for your speakers. Maybe adding harder or softer stones does have a profound effect on the bass energy it self.

I meant to say is adding hard or soft stones under your speaker. This usually does have a good effect on the amount of bass you can tame but this technique is limited. As some other posted also suggested get bass traps to further help on this aspect. Too much room energy will effect the midrange a lot more.

It is also not advisable to put any forms of diffusers behind you. This is more ideal on the side walls in a symmetrical layout.

I have experimented with Skyline diffusers over the listeners head and this worked out very well. just be sure not to over do the room with too much diffusers. It is also advisable to use a different mix of absorption materials instead of using the same one all over.

In a Typical studio i have seen the setups done with multiple different absorption materials.