There may have been some RFI or noise going on, which causes brightness, in the demo you had. ... Long story, but the thread is full of stuff about getting the M-Scaler and DAC right. WAVE Stream, Stone, and Storm cables are well regarded for this. They act by preventing anciliary noise getting into the DAC analogue circuits, and causing brightness.
Secondly, the brightness you heard may have just been adjustment related. I certainly noticed with every upgrade from Mojo, to Hugo 2, to TT2, some initial perceived brightness. However it was just more detail and spacialisation. Like when bass notes suddenly become more detailed, they can sound brighter.