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

Room treatment for reflections is high up the list.

Electronically, it would be to increase the resistor value in the tweeter’s high-pass filter on the crossover (e.g. go from a 2ohm resistor to 2.7ohm resistor). This would reduce the level of the tweeter by a db or so and leave everything else the same. But you’d need to be OK removing the drivers, crossover, and soldering. Tweeter crossover capacitors could also be swapped out for more boutique brands with a warmer sound signature. A couple of resistors would just be a few bucks. Boutique caps could get much more expensive depending on brand and values needed.

Otherwise, cable switching (or biwiring) could give you the normal audiophile "solution" to tone controls!

One might also consider a dsp solution for room eq.

A sub does not "solve" bright treble; it would rather change presentation if low end is lacking. Not the same thing.

TBH, everything except room treatment and the resistor change would be of dubious value for money in my eyes.

Placement along with room treatments....   I recently moved, my last room was a nightmare.   New room has better dimensions but more importantly a thick carpet.

I used my Veleodyne sms 1 to measure the new room's low freq response and it was damn near flat... the sms applied very little eq...   the old room has a serious room mode centered at 40hz that really muddied the sound ,  the sms 1 used a lot of eq for the old room, -9dB  and unlike the old room, the new room has no clap echo.  

This system has never sounded better and the only thing that changed was the room....  I know its probably not possible to set up in another room but my experience really demonstrated how much the room and it's properties play into the overall sound .  

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.