silicon pop culture!

I think my TR has always had a quirk that has bugged me for a long time.  When I am playing a song, especially if I have headphones or external speakers attached, the song volume will start out at one volume level at the beginning, but the volume of the treble will instantly drop as soon as a new instrument, especially a bass instrument, starts playing for the first time in the song.  It almost seems as though the computer is siphoning off energy from the treble to play the bass. 

This is terribly annoying because, just when the song is supposed to get more energetic and exciting from the introduction of drums or added bass instruments, it get the opposite effect because the overall volume of the song drops.  It feels like someone took the air out of the song.

Please advise.