Hi M
, I posted a thread starter about this already.
In order of price: (Bear in mind this is for a domestic situation as opposed to a dedicated HiFi room, bass being the single hardest area to control).
1. Ceiling insulation encapsulted in a frame with decrotive cover material attached to walls, min 200mm thick.
2. Rock wool as above for better attenuation.
3. MDF 20mm square rods of varying lenghts randomly affixed to a back board of around 1 mtr square so not to be too dominant(but a bugger to dust!) for dispersing mids and highs (no good at bass unless > 500mm long. Use a mirror at the seated listening position to locate when speakers seen in reflection.
4. Thick 'rug' carpet or tapestry hung on walls.
5. Stage quality pleated lined velvet curtains to one quarter of the room at speaker end using 4 inch deep pleats.
6. Home built 'bass traps' - lots of info about how to build on web placed as min in 2 corners behind speakers.
7. Thick natural fibre (wool?) carpet.
8. Large absorbent couch with large cusions.
9. change your speakers for those that are designed to work in the room volume.
10.Digital equalisation.
First you'd need to work out what frequencies are being troublesome, usually they are around 40 to 200 hz in the bass and 2 to 5k in the top. Once established you can then set to work attempting to change the room's balance. Forget 'acoustic foam' it looks horrible and only affects the mid and top, it's also overpriced by around 20 times it's true worth.
Just my 2p's worth
V