Definitions for "boxy"
1) Characterized by an "oh" vowel coloration, as when speaking with one's head inside a box. 2) Used to describe the upper-bass/lower-midrange sound of a loudspeaker with excessive cabinet-wall resonances.
Having resonances as if the music were enclosed in a box. Speaker cabinet diffraction or vibration. Sometimes an emphasis around 250 Hz to 500 Hz.
resembling a box in rectangularity.