How does unavoidable 'furniture' affect the room response?
How does unavoidable furniture, even things like amps, guitars, bass, et cetera, affect the acoustic response of the room?
It's quite hard to say “this” does exactly “that”.
Rooms are way too unique to make that kind of assessment.
And small rooms with a lot of clutter in them get so complicated so quickly in terms of their response that it's just really hard to pick apart and understand what exactly does what.
But let me give you a general guideline to assess the actual impact of furniture and other things in your room.