What strikes me most is that between the computer, its power supply and power cord, the "regular USB cable," your high powered "hybrid Class D" amplifiers, and the QB9, you've got a lot of things in the setup that can generate RFI and digital and switching noise, and potentially couple it through the air or the power wiring into other parts of the system. With consequences that are unpredictable but very conceivably could affect perceived brightness.
Some things to consider trying:
1)Put a shielded power cord on the computer, or on its separate power supply if it is a laptop.
2)If the computer is a laptop, see if you perceive any difference when running it on its battery vs. its AC power adapter. That would provide insight into whether switching noise from the power adapter is contributing to the problem.
3)Try different lengths of USB cable. Keep the USB cable as far away as possible from components and other cables. Consider upgrading that cable.
4)Plug the computer into an outlet that has separate AC wiring than the outlet(s) used for the rest of the system.
5)If they are not already present, install power conditioners. One for the computer (to minimize noise it may inject into the power wiring), and one for everything else (perhaps excluding the amplifiers).
In saying all this, I'm assuming that the root cause of the problem is not the speakers, speaker placement, or room acoustics. If there is reason to suspect that assumption is not correct, I would address those issues directly, and not try to band-aid them with cables or tweaks.