First, check the connection from the amplifier to the speakers. For speakers with a bi-amping connection, the connection to the high-midrange connection may have come loose.

Another possible cause is a problem with the tweeter, which is responsible for reproducing high frequencies. If the tweeter is not working properly or is damaged, this can cause the speaker to sound dull and muffled.

The sound setting on the amplifier may also be the cause. If the bass or the low frequencies are set too high, the high frequencies may sound muffled in relation to the low frequencies. A defect in the amplifier can also be a cause for the impaired sound.

If the room acoustics are not optimal or if the loudspeaker is placed incorrectly, the sound may also be dull or muffled.