Thiel CS2, fairly new... uhm, Correct me here guys, didn't thiel make those in the 80's?
Am I mixing them up or something here?
I'm throwing in the towel here, I can't twist my brain around this.
Everything tells me that this should NOT sound bright, it shouldn't

Oh my brain hurts...

Are these Cs 2.4's maybe? they can be somewhat close to bright...
How about this:
Try to put a sheet of toilet paper in front of the tweeter, if that tames the brightness, the speakers are indeed bright! (it will filter down the hi freq's some) try with and without.. ehm...
Try that!
Play....ehm.. Aguilera "Can't Hold Us Down" from the album "Stripped" should be a good test this.
Listen for sibilance and the floating shimmering going back and forth between the speakers.
This song is "neutral to warm, with no added bass power, and a very neutral treble, but if the speaker is bright... ouch!!!" you'll know!
The brightness could also be a toe-in issue... angle the speakers more outwards maybe?
Imperial