Great suggestions and resources here for treatment. Ken, I'd have to agree that glass behind your head is a big no-no for optimum sound, but again, if it ain't broke...
That said, yes, get rid of the glass and replace it with some sort of diffusion. I put up some heavy curtains and have a project to build a DIY diffusion panel. The OC 703 panels that are mentioned are absorptive panels and NOT diffusion. In addition to the great resource for the inexpensive covered 703 panels, they are also available from companies that deal in insulation (these are Corning insulation panels made from dense foam). A five pack of 2X4 panels should be a bit over $100, or rather they are in my area. These are normally not the foam carried by Home Depot and other large chains. It is also not good to have glass or hard surfaces in between your speakers on the back wall, and here again a combination of diffusion and bass traps may be the way to go. Absorption can also be used, as on the side walls and ceiling. There are numerous threads and resources for room treatment. I would imagine, given that you are happy with the sound now, and by the fact that you are listening nearfield, that you may find that limiting treatment to only one or two points may make a big difference. It is a great suggestion to experiment before jumping in head first by removing the offending glass and replacing it temporarily with a thick foam cushion or heavy blanket if you can find some way to suspend either behind you. Play some music you are very familiar with before and after and see if the difference that makes is worth the effort and expense.
Again, I would not dwell further on it if you cannot hear a difference, and would go with that great old children's song lyric that I remember from Romper Room ...If you're happy and you know it clap your hands! Hey, was that an echo I heard just then?
That said, yes, get rid of the glass and replace it with some sort of diffusion. I put up some heavy curtains and have a project to build a DIY diffusion panel. The OC 703 panels that are mentioned are absorptive panels and NOT diffusion. In addition to the great resource for the inexpensive covered 703 panels, they are also available from companies that deal in insulation (these are Corning insulation panels made from dense foam). A five pack of 2X4 panels should be a bit over $100, or rather they are in my area. These are normally not the foam carried by Home Depot and other large chains. It is also not good to have glass or hard surfaces in between your speakers on the back wall, and here again a combination of diffusion and bass traps may be the way to go. Absorption can also be used, as on the side walls and ceiling. There are numerous threads and resources for room treatment. I would imagine, given that you are happy with the sound now, and by the fact that you are listening nearfield, that you may find that limiting treatment to only one or two points may make a big difference. It is a great suggestion to experiment before jumping in head first by removing the offending glass and replacing it temporarily with a thick foam cushion or heavy blanket if you can find some way to suspend either behind you. Play some music you are very familiar with before and after and see if the difference that makes is worth the effort and expense.
Again, I would not dwell further on it if you cannot hear a difference, and would go with that great old children's song lyric that I remember from Romper Room ...If you're happy and you know it clap your hands! Hey, was that an echo I heard just then?