Like the poster above : room acoustic and acoustic homemade devices...
After that in the opposite move of the OP i disconnected my subwoofer because the bass was so well improved i dont need it...It is the reason why i also put my 8 headpgones in a closet.,.. 😁😊😎
Coordination of subs can be a great upgrade for sure like the OP suggested , but in my small room i dont need it at all...
Acoustic is the KEY UPGRADE for all other upgrades... And after acoustic like me perhaps you will forgot about any gear upgrade....