Thanks for all your responses. They are very insightful. What I exactly mean is that in comparison with the lower midrange/upper bass the midrange and upper midrange/treble are deprived from a certain richness in sound (= bloom). I hear enough bass and mid bass, it's even too much some times. But upper midrange and treble should sound rich and with plenty of "air". I'll experiment with different speaker positions first.
Chris
Chris