As Tvad suggests, you may find there is nothing you can do to compensate for the component that is causing the brightness. That would be a good place to start.
Having said that, I once owned a Bryston / Thiel combination and loved it except for the brightness. Once I switched my speaker cables to MIT, everything came together. If it's easier to switch out cables than to replace the source of the brightness (or if you like everything about the source except for that), try some MIT's to see if they make a difference -- you can normally try before you buy so there should be no risk if you find the right dealer. I've had excellent luck with Joe Abrams, but don't forget your local dealers.
Having said that, I once owned a Bryston / Thiel combination and loved it except for the brightness. Once I switched my speaker cables to MIT, everything came together. If it's easier to switch out cables than to replace the source of the brightness (or if you like everything about the source except for that), try some MIT's to see if they make a difference -- you can normally try before you buy so there should be no risk if you find the right dealer. I've had excellent luck with Joe Abrams, but don't forget your local dealers.