My rig - Win7, latest GPO, Sonar 8.5
My piece has a couple of tracks using Cakewalk's Rapture synth, plus tracks using GPO - Section Violin 1, Section Violin 2, and Viola. Another instance of Aria has the three solo cellos, and two solo double basses.
When I make any change in the violin or viola parts, my level in the cello and bass parts drops by more than half. All of the sliders in both Sonar and Aria are up. Inspecting the velocity and volume controllers show them at the appropriate level.
To get my levels back, I have to go into Aria and reload the patches. Then everything plays correctly until I make a change in the upper strings. Obviously something I'm not thinking of.