What are the limits of room correction/ treatments ?

Stillpoints Aperture panels are the best and do wonders in any room.

Appropriate room treatment can make all the difference!  suggestions:
1.  Treat all wall intersections.  I have had Michael Green's Roomtune devices in mine for many  years.  
2. If the bass seems a bit much, bass traps may help.
3.  I like absorption behind the listening area so you aren't subjected to sound bouncing off the wall behind you.
4. Diffusors behind the speakers can yield substantial benefits.

If you Google, you can find a variety of products that may get you where you need to be.

Good luck!

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.

YES!

Room treatment fist, EQ/Room correction second. GIK Acoustics, great advice and highest value products.

Room treament enables EQ, especially in the bass.

Best,

Erik

There are internet sites that have room mode calculators, displaying in 3d where standing waves appear in the room, which is determined by the dimensions of the room. You put your room's dimensions into the calculator, and a 3d drawing appears, showing where in the room nodes are located. If you place your speakers and listening position where no such nodes appear, you have minimized the resonances of the room itself. Room treatments are therefore minimized as well.