Finding source of sibilance

I've recently put together a few new components. A Clearaudio Concept TT, the Clearaudio MM cartridge, and the Pro-Ject Tube Box DS. Together, I'm not loving the result. The highs sound actively bad and sibilant. Is it possible that I installed the cartridge incorrectly, or is this just not a good setup? I would assume the TT and cartridge themselves are meant to be together. Any help would be amazing!
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First question, as slaw is saying, does your system have sibilance problems when using sources other than the turntable?  If so, more information is needed.  If not, you might check the alignment
or VTA of your cartridge, that could be a cause of the problem--in my setup, for example, if the cartridge is set up so that it is not parallel to the platter and it is instead leaning forward (i.e., the back is higher than the front, looking from the side), the highs are accentuated. 
Sibilance can be caused by even a small degree of cartridge misalignment. But it can also be caused by the stylus being attached to the cantilever slightly "off" by it's maker, or even the cartridge being run with too little tracking force. A qualified turntable setup man in your area is a blessing! If you provide your location, perhaps someone can recommend one.
Assuming your system is set up properly (im sure it is) how long have you had this equipment? It takes roughly 100 hours of burn in time before things open up and sound right.Try not to jump to worse case conclusions right away, its probably something simple. MM's like to have the correct capacitance set, so make sure you include the cables capacitance too, this can be 50pf or as high as 250pf! try and keep cable short (2 ft) to reduce this.

Matt M
The highs sound actively bad and sibilant. Is it possible that I installed the cartridge incorrectly, or is this just not a good setup?
If you're not sure that the cartridge is properly setup, then it's likely that it isn't. What tools did you use to align overhang, tangency and VTA? Did you align the cartridge body itself, or did you use a mirrored gauge and align the cantilever? Answers to those questions will help determine what might be going on here.
Clearaudio states that these tables arrive setup in a foolproof way so that no further adjustment is needed once it leaves the factory. All I personally did was attach the cartridge. Nothing else in my system has this issue, i.e. digital sources. I think I may have to find somebody in NYC that can have a look at it with a professional eye. Or, I simply need to wait until the system has settled through running it in for longer.
I've never had a sibilance issue that could be traced to cartridge break-in.
It is not clear to me that you know the issue is from the turntable.

Do you have another source which does not exhibit this?

It could also be the phono stage perhaps? It’s the Pro-Jext Tube Box DS. Digital sources through a streamer > DAC sound good.