I would start with the amp. Your ARC preamp deserves something better than an the Adcom. I sold that Kenwood turntable when I sold audio, it is an excellent table so I would keep that. B&W speakers are bright, I personally don’t care for the line at all, but many do. Have you verified where the sibilance is coming from? If the digital also sounds harsh, then it is either the amp, pre or speakers. I wouldn't throw any money at the system until you have that figured out.
system synergy and problems with SQ
I have been trying to build a system out of new and used components and have persistent problems with unsatisfying sound. Can you help me figure out what is wrong?
The system:
Old Kenwood KD 5070 turntable with Ortofon Blue, Bellari 129 preamp with Gold Lion tube (did some tube rolling and this greatly improved the harsh highs), Audio Research LS25 mkII preamp, Adcom GFA 555 II power amp, Speakers either Celestion ls700 se or Bowers and Wilkins 803 matrix series II. For digital, Qutest DAC fed by Audirvana/Qobuz on a Mac Powerbook.
On good recordings the Celesions give a wonderful sound stage / instrument separation with vinyl. Their rendition of instrument timbre is really captivating. The B and W are more detailed in the highs, so provide realism that way, but have no rolloff and are harsh on a lot of records. Their soundstage is OK but not as good as the Celestions. Digital sources pretty much the same differences, but imaging is not as impressive with either speaker. The problem is that the B and W are often harsh, the Celestions are amazing for recordings with a few instruments but can’t handle full orchestra at reasonable volume. Sound is harsh. Is it the vinyl and the tubes that are producing the imaging magic? Can the Celestions handle volume and many instruments if I change something? Will a new phono preamp fix this? Thanks a lot for your help!