Speakers will change response as heat builds up in the voice coils --- you get thermal compression after some minutes of play as the coils inside the drivers warm up. As you play louder the compression will get worse...anywhere from 0.5 to 3DB or more in compression can be expected (enough to be plainly audible). On passive speakers it will also affect the way the passive crossover behaves as the filter will drift slightly.
This effect is volume level and speaker design dependent.....it might possibly be the source of your problem. Certainly, speaker thermal compression could cause the sound to go from a bright sound to a duller sound.
If you have ever played a track very loud (at the beginning) and then found yourself increasing the volume a few minutes later then you have probably experienced speaker thermal compression. (Not to be confused with an old Vinyl trick where the mastering engineer deliberately increased the level at the start of the track to help it sell...as loudness is often interpreted as better sounding)
G'luck!
This effect is volume level and speaker design dependent.....it might possibly be the source of your problem. Certainly, speaker thermal compression could cause the sound to go from a bright sound to a duller sound.
If you have ever played a track very loud (at the beginning) and then found yourself increasing the volume a few minutes later then you have probably experienced speaker thermal compression. (Not to be confused with an old Vinyl trick where the mastering engineer deliberately increased the level at the start of the track to help it sell...as loudness is often interpreted as better sounding)
G'luck!