effectively you have created anothe cabinet arund the speaker and this is why they boom, not a lot you can do really except fill the gap with sound proof material ideally you should have used in wall speakers not mounted free air speakers in a wall room EQ may help but will cost more than doing it properly.
If you read the instructions on a lot of quality speakers it will state something like, can be wall mounted but leave 0.5 - 1m from corners, in your case if looks like 1 inch away. All speakers unless designed for in wall fitment expect to see free space all around them.