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

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.

Hi, You should look at a ZROCK2 from decware, go to their site and read about it. It just might fit the bill!Good Luck, Tish

Are the speakers revealing other issues further up the chain? Those big transistors or a poor source? Solid core speaker cables will help. 

Thanks everyone for your replies. They are varied as one would expect.

My room is far from ideal, but I don’t think it’s having a major impact on this problem. The listening area is carpeted and has a fabric sofa and chair and the house has an open floor plan, so I don’t think it’s excessive reflections. This suggests to me that maybe subwoofers would help.

I do agree that room treatments could improve and maybe eliminate the brightness, but that would probably be the most difficult way to attack the problem. It’s something I would like to do over time, but it will require a lot of time and expertise.

There have also been suggestions for cables and the controversy that comes with them. I’m using Analysis Plus Clear Oval speaker cables. Don’t let the name fool you, they’re not bright cables. I don’t know how they came up with the Analysis Plus name, but they’re just good quality copper cables using their proprietary oval construction methods (to reduce skin effect, according to AP). When I get the right gear together I want to experiment with cables with The Cable Company, but I’d rather not try cables at random, unless someone has experience with these particular speakers.

That brings us to upstream gear. The D’Agostino Classic Stereo that I have is a powerhouse amp, 300 watts at 8 ohms 1,000 at 2 ohms according to Stereophile. I don’t need that power with the TADs and changing amps would probably be a good place to start. Would a Class A amp like a Pass or Plinius or Accuphase relax the mids and highs a little or just give me different sounding bright speakers? I don’t think the D'Agostino is a bright amp but I haven’t heard it described as laid back or refined either. Tubes are another option. Any suggestions for a change of amps?