When I was younger, I played a lot of guitar. Needless to say, my room was often very messy with clothes all over the floor, this made a huge difference to the sound. I'd spend ages getting a nice tone on my amp but then, when finally forced to tidy my room, the amp would sound different. This was my first experience of how drastically the room affects your listening. As you know, accurate monitoring is essential, which is why recording studios have invested tens of thousands for sound treatment. But what stuff in your room are actually the culprits?
Your first issue isn't the contents of the room, it is the walls. Sound waves reflect off walls and so can interfere with themselves, normally for regular listeners. this isn't a huge issue and isn't perceived but we want a flat response over all frequencies. The thing with interference is when there are standing waves that massively increase the volume of certain frequencies.
Standing waves occur when the room is the same length, or a multiple of the length of a wave. This means, when it bounces off the wall, the peaks line up and the volume is doubled.
This phenomena is visualised with water waves, where
the waves become more prominent as they are constructively
interfering with each other.
Standing waves aren't the only issue however. If you have a room that echoes like a church, you are going to have a hard time finding a suitable reverb for other rooms. You want to minimise the reflections, for sake of clarity and understanding what the "dry" sound actually sounds like.
So how do we fix the walls? There are clearly going to be standing waves and rooms naturally have at least some reflection and reverb. The answer depends on where you are. Ideally the room should not be square or have 90 degree corners, short of that though there are a few options.
Certainly out of my budget.
Soft things are great, even installing a sofa makes a difference, especially if it is on the back wall facing the speakers. Thick curtains or drapes will help too. Definitely get some cushions and carpeting.
Acoustic foam treatment panels can be cheap, especially if made by yourself and is easy to stick on the wall, figure out the best positioning for it. These really do make a difference, especially to a large flat wall. You will learn to recognise when they are absent (e.g. fell down just by talking in the room).
If you have any hollow furniture, especially if you have placed your monitors on it, it can resonate causing an unreal response. In fact, it's not just the furniture but other small things to. Resting your monitors on cardboard boxes? Probably a bad plan. Even if it's not hollow, it can resonate, which is why you should have some isolation pads under the monitors.
These pads dampen the vibrations that could resonate
through the table.