@asctim Sorry missed that you meant with headphones!! Hahaha. :)
I think you can always deaden a room too much, but generally speaking I have found that room treatment works in the plane you apply it. That is, if you treat the floor and ceiling it adds height to the image. If you add behind the speaker and listener it adds depth.
I did a whole thread on first reflection points which was misunderstood I think by many readers. I don't think room treatments work very well at all until you reach a certain critical mass. If you ONLY treat first reflection points I can barely hear any benefit at all. It is only when the room overall has enough treatment to quiet it down that the 1st reflection points matter.