What you are describing could be caused by high capacitance speaker cables, which causes some solid state amps go into oscillation. What speaker cables were you using?
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I made up some temporary speaker cables out of 10/4 SOW (had some lying around) when I moved because I needed more length. Did have a sibilence issue and I do have dual bridged SA100's. Ordered some teflon insulated speaker cables from Carl at Ultimate Cables and that fixed the problem. To tell the truth, I thought it was a room issue and was quite surprised.
Some years ago I had a Plinius SA250 Mk IV together with its Preamp and found the midrange presentation recessed to an extent that the bass sounded impressive, but the highs sounded brittle and sibilant as you describe. After 3 months I could no longer live with this and happily went back to my Perraux PM 1850.
I use Kimber cables, recommended by the speaker maker, not necessarily the amplifier maker. I do not believe them to be high-capacitance.
The SA 100MKIII is very impressive in its ability to capture nuance and reveal internal lines in complex orchestral passages. It has great bass (delineation and control), fine dynamics, and very good timbral accuracy throughout the orchestral range. Add a huge and very broad image, with great height and depth, ignore the high running temperature and the heavy electrical drain and, if it weren't for this zinginess I've described, I'd be quite happy.
I might just experiment with speaker cables. Changes of interconnect have not helped.
I perceive the area of principal coloration to be in the sibilance range and it seems to add a slightly whistle-like character to the sound, especially at high levels. On some (maybe rolled-off) recordings, this emphasis can be attractive, enlivening the string and brass sheen that occur naturally; but on most cd's and lp's, the effect is not to my taste.
I experienced something similar to this during my recent audition with the latest Plinius Hiato integrated amplifier. My product audition experience HERE goes into more details.
I own the SA-100MkIII. Instead of experiencing sibilance and a slight tilt in the high frequencies, I got a smooth tube-like midrange with a treble that's just right. It depends on the speakers you are driving with the Plinius. The Plinius has a dark character and with the equally warm Sonus Faber Grand Piano speakers, the sound can get pretty dead as both treble and midrange were kind of shut-in causing music to lack sparkle and dynamics. With more neutral speakers like the PMCs, the sound opened up a little albeit still a little to the warmer side of things. With brighter speakers like the Proac Tablette 50 Sigs, the matching was better. In all cases, I did not find the SA-100MkIII to exhibit high frequency anomalies with most range of speakers even the brighter ones. Not too sure if the Shahinian Hawks are very bright speakers. You can try a warm tube preamp or warm cables to ameliorate the high frequency anomalies. Another possibility is the acoustics of your room. If your speakers are close to bare walls, chances are the anomalies, especially high frequencies are caused by the room acoustics due to reflection of sound waves off the walls. Some absorption will solve this problem.
FWIW I used dynamic interconnects to boost the mid to high frequencies on the Plinius amp. I have 2 sets of XLO Signature 2 cables throughout from CDP to preamp to the SA-100MkIII, and my preamp is lean-sounding.
I have had Shahinian Diapason and drove them with Plinius SA-102. Wonderful top end, one of the best in SS amplifiers I ever heard. I sold my Plinius partly because it could not provide the adequate bass and replaced it with Musician III which appear on market at that time.
Check cabling I would add to others suggestions.
I was fortunate enough to have my SA250MK4s modified in New Zealand where I reside which was researched by the man who was the main force behind the design of Plinius amplifiers prior to his departure in 2005. I was then able to make a side by side comparison with a SA REF which I had for 2 weeks. The modified SA250MK4 performed to a level beyond that of the SA REF. This was also confirmed by another Plinius SA250 owner who was present at the time. Mod is $900NZD and ensures long term reliability and improved sonics. Any queries email: email@example.com