Hi, Jonesey!
I checked my project and the master bus is at -6dB, and I also inserted a VST reverb and a FX Chain from Cakewalk's mastering presets on it.
However your post made me check in the console view and I believe I created bus sends [for virtual instruments] but didn't pay attention on what I was doing: the buses out were addressed to 'surround main' (don't know exactly what it is!).
I readdressed them to master bus and now piano sounds lower, but at least it seems to be what was really going on.
Thanks for the help!