Big coffee tables, regardless of material, are less desirable for HT. Audiot is correct, but I will say that while I prefer the sound without the table, it made only a slight difference putting it back in my room.
Try and get the smallest table you can, glass being the least desirable. Also consider increasing the toe-in of your front speakers - start in the center seat, and lean side to side until the sound is equal at all 3 seats.