Must have high quality upstream electronics. I've listened to Thiel CS6 at a friends w/classe mono blocks and it sounded bright. Room setup & speaker placement could have been partly to blame.
My CS2.4 Speakers sounded bright before I got BAT VK-600 @ 300w per channel.
Even recently my new PASS X150.8 amp isn't as enjoyable to hear as the Balanced Audio Technology. Something about not only the high frequency but a glorious sweet sounding full midrange. The pass is a very fine amplifier and does have its own secret sauce but with another speaker perhaps...