From what I've heard, the
*GRADE UP music encompasses the tracks that include an extra sound channel not normally possible with the 2A03 sound chip of the NES/Famicom.
Let's compare:
Temple of Darkness (original)
Temple of Darkness (Grade Up)
Notice the extra octave in the lead?
Assault: Stealthy Devil
Assault: Stealthy Devil (Grade Up)
Can you hear the higher pitch square wave?
Also, just to note: not only is the Brave Wave album in mono, but so is the NES hardware. With that in mind, this whole album is enhanced. But someone really went to work with that bloody Malth theme!