As this appears to happen when there is a single stringed instrument playing, it may be an indication that the cones of your speakers are becoming soft and possibly, losing their rigidity.
To test this, play a recording of a simple, unplugged and plucked, stringed instrument. Say a guitar, or harp, and gradually increase the volume. If the sound loses its crispness, your Cones have deteriorated, usually caused by playing louder than their designed capability.
It used to be possible to re-cone large (8", 10", 12" & 15") speakers but, with the cost of labour these days, it's probably cheaper to learn to live with it until you can afford to replace them as most commercially made speakers are in sealed cabinets