system synergy and problems with SQ

Let’s back up and go over how the DAC sound playing the same stuff?
If that is acceptable, then it is sibilance that you’re hearing?
If so that is often resonance and cart, arm and head shell are players.

Your part of at “reasonable volume” indicates tracking or resonance. If that is the causal mechanism, then it will not be fixed with a phono stage. It would be fixed by TT set up, and/or a different cart.

Let’s see if the sound through the DAC is OK first… ?

thanks for your help. it is hard to find a fair comparison, but I can look at the same recording played on quobuz versus an lp. when I do that, the problems with the b and w are much worse. both large scale and intimate recordings are harsh unless the highs are really well recorded (I suppose that is the cause).  The celestions still are OK to listen to (not harsh) but don't have the soundstage of the vinyl. As with vinyl, they have a hard time playing full orchestra loud.

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.

Does the preamp have a headphone jack?

I think an ARC/Adcom/B&W would not be a great combo. That tilts to the lean and dry sound. The speakers can be great, the Adcom is not that bad, but the ARC pre is flattening things out. I'd go with a Cary pre or another preamp known for their liveliness, maybe a Conrad Johnson. Then I'd go amp shopping. 

No headphone jack. That would be a good test. I am surprised the ARC coukd be the issue as it is a tube preamp. I have a simple volume knob from Schiit that I can use to run without the pre, and will check. Also tried a Classe Sigma 2200i integrated, which made the problem much worse, I guess because it is a class D. Will do some more process of elimination as people suggest. Thanks much for your help. 

No headphone jack. That would be a good test. I am surprised the ARC coukd be the issue as it is a tube preamp. I have a simple volume knob from Schiit that I can use to run without the pre, and will check.

^that^ could exonerate or point to the preamp… so it makes a lot of sense to try it.
(Tubes sometimes need replacement)
And it also takes out the TT, cart, phono… so it is a double dose of being able to get closer to finding root cause.

Curious...what do you have the gain set to on LS25? Also, have you taken the Bellari out of the mix and just used the internal phono on the LS25?