Sibilance in recordings: your experience the same?

Annoying sibilance is most likely systemic...not break in and such.
Do you have anything (power conditioner) between the 'box' and your system? I didn't see any reference to one.
Check the wiring to make sure that you have everything wired correctly, and that you're getting 120V from the wall, and that it's clean power.
This is an all day project, one best done with a step by step process, committed to paper beforehand. When you come up on the problem,(polarity, whatever) you'll join the 'flat forehead' society which I founded many years ago.
My oldest rule of audio...."If something sounds wrong, something IS wrong..."
Don't try to rationalize or make excuses. If it sounds wrong it IS wrong...you just have to find out what that wrong thing is.
Good LUCK!

Sibilance, to me is the most annoying of virtually all system flaws. I remember my first foray into High End audio was disappointing almost entirely, because there was virtually no female singer who didn't sizzzzzle into the mic, or so I thought.
Extended HF, with no regard to interconnects, speaker cables, GOOD solid state, (which back then was an oxymoron), a cart for the table which could actually track correctly. That sibilance unearthed a vast multitude of problems.
"What's New", by Linda Ronstat was such a dissapointment for a long time because of this. The music, I loved, the rendition of her voice, not so much.
They obviously peaked her voice in the upper ranges; why, to give it more 'presence' I suppose.
There is a device which produces 'tube like' distortions, known as, and I'm no doubt wrong on this, "Aphex Aural Exciter", which introduces tube like distortions onto the recordings. Many female vocalists, Barbara Streisand being, at least at one time, the most visible, LOVE this effect.
In designing some speakers, one of my principal goals was to have NO SIBILANCE ADDED. Sure some recordings sound that way, but a speaker with just a few db of spike in the upper regions, and in the wrong bandwidth, can send most of us running from the room. Again, and I've said it before, a good theology for speaker designers, could be "The best tweeter I ever heard, I didn't."
This should be viewed as, HF's sound electrostatic, not 'dome'.
I'd personally rather have a slight roll off than that nails on the chalkboard sibilance.

"I'll talk, I'll talk, God, please make them stopppp!!!!!!"

Without any scientific data to support this, I would say that, while I agree that sibilance is a real artifact, that it's not limited to simply microphones. The reason I question this is, why does it manifest primarily and while not exclusively, at least mostly in vocal recordings.
Massed strings, which because of the focus of thier collective bandwith, would seem to be very prone to this same effect, yet I don't find it in the same amounts.
That, plus as I listen to the remakes of Winston Ma's works, recordings which sound so unbelieveably sibilant in thier original forms, sound extraordinarily less so after he performs his 'magic'. Winston told me personally, that his process is one of (of course many more things than this and I am simplifying) removing layer after layer of noise within the recording.
The harmonics of say a female voice, an octave, two, and three above the dominant in that female voice reside in the 1khz, 2kz, 4khz frequency bandwidth. This, not too coincidentally is spread over the range of many mid to HF driver crossovers.
With time and phase 'smear' in this region, coupled with great amounts of noise in SOME recordings the result is a heightened distortion level.
As to tubes smearing this information, I don't think that's it. Tubes, by most, even solid state designers are by consensus thought to have better low level resolution. That greater resolution may simply give more information over the entire spectrum allowing for a fuller more complete therefore less sibilant sound.
I offer up the one theory, crossover issues in dynamics, as one thought after living with the Sound Labs for quite a while and not having anywhere near the same level of sibilance introduced. Also, the Quads, and even the Maggies full panel units. None of these panels, to my ear share in these sibiant issues to nearly the same degree.
This group of theories has, again, no proof offered, and may be completely wrong. But given hundreds of hours, make that thousands, of listening, it seems that some or all of these may contribute to the annoying artificacts we call sibilance.