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

If you want to experiment with tube components investigate tubed preamps.A smooth,warm sounding preamp such as Conrad Johnson units for example might add just enough sweetness to tame that brightness.Then experiment with various tubes until you get your system tuned to your preference.

Room treatments. 

Toeing then out out a bit to get to where the tweeters roll off could help too if it does not mess with other things too much. Just have to try and see. It is free. 

Jim Smith Book on better sound, felt, revisit excessive toe in and or tilt, room treatments and then hardware changes

No joking the Zobels networks I have advertised in Loudspeakers will reduce the upper frequency noise and your system will sound better .if not  money back .i just spotted this in the forum .
that  is the easiest way ,or you can solder  in new capacitors.

I have a pair of VMPS RM40 ribbons speakers that have 40" of ribbons drivers per speaker. They are very detailed, transparent, and dynamic with good bass. They have a tendency to get a little thin and maybe hot sounding with some recordings. After a couple friends came over with their ICs they liked a lot, we listened to my system using my Darwin Ascension and some Amadi Maddie Signature ICs that I have used for a few years and really liked. Both of these have silver wires. I’ve never thought they were etchy or hot in the highs, but when we switched to Teo GC ICs, we were all in agreement that the sound was much improved in the organic sound of live instruments and made a big difference to the overall sound. They also were every bit as detailed as the prior ICs. Later in this year, Doug Schroeder of Dagogo online reviews, told a couple of us about the doubling up of Teo ICs to each component and the 3 of us did the same listening test at a later date to hear the difference. I had bought a couple pairs of Teo GCs to run from the CD to pre and from the Phono to pre. The result was even more impressive than just switching to the Teo GCs. There was VERY noticeable improvement in ALL ways compared the single. The sound of the RM40s was now about as good as could be desired and definitely the best I’d heard from these speakers. And I used to be a VMPS demonstrator for the last 5 years the company was in business.
These cables could be just the ticket. Whatever you end up doing, the doubling up of ICs, now called the Schroeder method in the Forum, will make a quantum jump in the performance of yours or any other system. The Teo’s will take them in the direction you want to go. I now have a pair of Vapor Audio Joule Black speakers and the Teos still sound fantastic with these speakers, and the Joules are much more organic and natural sounding than the VMPS’.

Bob