more important then buying new gear and most ignored by all is room treatment. Your idea of just stuffing is in the right direction but off a little. Place it tri corner v shape with an air gap"cavity". Also books are great for both damping and diffusing along with lps.
Absorber behind corner bookcase
I recently moved a bookcase against a corner. It fit well, but leaves a triangular space behind the bookcase. Would putting a rolled up bag of insulation back there help with bass diffusion or would it matter? This is a corner where I can put some diffusion and hide it from view by the bookcase.