room treatment gurus, please

This my friend's problem. The listening room is 16 x 16 with 13ft ceilings. It was built in the 1980s. One wall has two sliding wood doors, 8 ft high by 12 ft wide total. These doors separate two rooms. This  door is 12 inches behind the head of the listener. The opposite wall is all drywall. One side wall has another sliding door with two glass panels for room entry, total is 6ft x 6ft. The other side wall has a wood and tile fireplace with an ornate mantle. The speakers are Sonus Faber Amati Tradition. All electronics are MacIntosh. He complains about poor bass, and I don't think his system sounds near as good as it should.

Now, I know a square room is the worst, and that one should cover all smooth surfaces (all wood and glass), including the floor, but that's about it.

So I ask. What should be covered? How should it be covered? If we switch the wall behind the listeners head, will this have a significant impact? How should the problem be approached?

TIA

Bill