What is the best way to tame a slightly bright speaker?

The TAD’s are had a little too much air from my time demoing them. Coaxial drivers have a very wide and uniform dispersion pattern and perhaps that’s why KEF rolls off the off-axis response of their drivers quite aggressively, whereas TAD chose not to, resulting in perhaps more detail but a sound that will be much more aggressive in the highs.

Your best bet is probably some form of multi-point measurement room correction software like ARC or Diraclive and tune it to a more rolled off target curve.

Right tube amp and cables ?

My friend has the same TAD speakers in a dedicated audio room that has  built in acoustic treatment and they are revealing but not bright. The suggestions to approach the issue from that angle are probably going to be an essential part of the solution. While it is true that a sub doesn't "solve" the problem of an overly bright speaker, one's perception of system balance is affected by the addition of a sub. My Tidal Piano Ceras without subwoofers can sound too bright with some material. When the subs are turned on, the system balance seems natural on the same material. Lastly, a very low budget endeavor would be to try Grannyring's "Acoustic BBQ" Western Electric 10 gauge speaker cables. I know it might offend one's sensibilities to pair such inexpensive wire with expensive equipment but I found that wire to be just what I was needing to deal with excessive treble energy after replacing my amp. His description of the speaker wire in the Audiogon advertisement is not hype.

It might be worthwhile to toe in or toe out the speakers depending on its on and off axis high frequency dispersion, in addition to taking reflections.  And, it costs nothing. To achieve tone controls with cables is unicorn hunting. 

I don't have any experience with this specific system but assuming your source is digital, in a (friend's) similarly revealing system moving to an MHDT Pagoda non-oversampling DAC rounded everything out. Poor AC can also manifest itself as excessive brightness so adding a power conditioner is also helpful in cleaning the unwanted noise. The latter was my own experience.