OK, it's clear a little clarification on acoustics is needed here. Not trying to mansplain, but just to provide some context.
1) Speakers provide input into the listening space. What you hear is the direct sound from the speakers plus the reverberant output of the room. This is why equalization, graphic or automated DSP is a mixed bag - it can only change the input to the room, not the output of the room.
2) In every room there is a point where the direct sound level equals the reverberant field level. This is called 'the critical distance'. Listening inside the critical distance is listening in the nearfield. Beyond the critical distance you hear mostly reflected 'far field' sound - mostly the room output. In a 2000 ft3 room, depending furnishings, the critical distance will typically be 3 to 5 feet (!). Extending that distance to hear more of the speaker and less of the room is a function of speaker angular coverage and acoustical treatment. This is why you see directional horns in recording studios and live sound, and why nearfield monitors like KEF LS-50s and venerable LS3/5a designs get lost in larger rooms - they are tuned for nearfield listening.
3) Acoustical panels (2" thick) absorb sound, mostly from 200 Hz and above. Diffusers, well, diffuse the reflected / reverberant field, normalizing the level and reducing 'hot spots. Placing either to reduce first reflections is the first objective of any acoustical treatment plan.
4) Below 200Hz is the province of bass traps. Since the fundamental frequencies of most musical instruments and the human voice are under 200Hz,the importance of bass traps cannot be overstated.
5) The acoustical requirements for a listening room, home theatre, and recording studio are all quite different, as are the requirements for recording a large symphonic work versus a small jazz quartet versus a multi-tracked pop or rock track. Home theaters in particular benefit from being the most damped, as you are replacing the entire reverberant field with the surround sound mix.
Overdamped 2-channel listening rooms, as noted, can sound 'dead', but left untreated they too often suffer from mid and upper mid-range 'glare' and lack of definition as well as boomy, uneven bass because the listening area is well beyond the critical distance, leaving your ears at the mercy of the sum of all room reflections and anomalies.
6) There are lots of online acoustical calculators to help begin the process of tuning a room for your specific requirements and listening taste. These are a starting point, not an end point. Also, there are numerous online advisors on the subject, some are quite skilled and others quite arrogant, most with a product or service to sell. Hey, gotta pay the rent.
Furniture is far too vague a term to generalize. Suffice to say a sleek italian leather sectional is at best a diffuser while an overstuffed Victorian fabric sofa is quite an effective, if not optimally placed absorber.
More than an other component, your room and it's acoustics will return more on your investment than any other changes you can make. A modest rig in a well controlled room will be more enjoyable than a mega-buck system in a poor room.