How do I smooth out violins?

I switched to a tube preamp using some great NOS tubes that rolled off the highs a bit, but still sound very open (RCA Blackplates). Strings were finally sounding very natural, and it was now easier to tell which cds were poor recordings.

Get better equipment or roll off the highs.

The problem with massed strings (and probably massed brass instruments too) being too overbearing often has little to do with frequency response and a lot to do with distortion. Solid state electronics have less distortion than tubes, but what distortion they do have is percieved by the ear as brightness and harshness and this is why tubes are still around decades on after becoming 'obsolete'. Tube distortion is simply less irritating.
Digital has its own distortions (known as 'aliasing' since the digital industry does not want to admit to having distortion) which is also perceived by the ear/brain system as brightness.
Tube preamps might well have the same bandwidth as a solid state preamp but not sound as bright/harsh simply due to this distortion issue, since the ear converts all forms of distortion into tonality. The reason the higher ordered harmonics (which contribute to brightness) cause this is the ear is keenly sensitive to them as it uses them to sense sound pressure.

I second rok2id4's suggestion about tracking down some good string recordings.  But before you replace any equipment as so many have suggested, try the Schiit Loki:  https://www.schiit.com/products/loki
Only $149, with a 15 day free trial and full refund minus only a 15% restock fee.  So if it doesn't help with your problem, you are out only $22 - way cheaper than replacing a preamp, cd player, cables, or getting a power conditioner.  Doesn't work and you are out $22, you can then then proceed to spend way more on one of the suggested fixes that also may or may not work.  Regarding your speakers, I was a long time owners of the BC-1, for which I believe your speakers are the 4th generation replacement.  While I've not heard the SP1/2e, based on how wonderful the BC-1s sounded, I would really hesitate replacing the SPs.

Depending on your speakers - it may be something as simple as playing with the placement and alignment of your speakers.

At the frequencies in the Violin range, human ears are pretty sensitive to reflected signals. Also, those frequencies "bounce" off of typical common wall surfaces pretty effectively. Furthermore, as some have stated - your speakers (tweaters in particular) may become more "beamy" at the higher frequencies (the way that various frequencies produced by your speakers "spread-out" within your room changes as the primary frequency content of the music shifts from predominantly bass to predominantly treble, and the spread of the tweaters may become narrower when the music shifts to mostly treble [as in massed violins] which would tend to focus more of the sound intensity to wherever the tweaters are aimed.

If your speakers are pointed directly at your ears (height-wise) and/or if they are pointed square or nearly so at any hard walls within your room - you may be hearing the tweaters TOO "directly" and/or reflections interacting with the original signal from the speaker. Changing the "toe-in" and/or tilting the speakers a few degrees may help to reduce this phenomenon (and it costs you NOTHING to try it!) The idea is to align the tweaters so that they are aimed "just a little" off from your listening location and at enough of an angle so that reflections do not bounce in direct opposition or parallel to the source waves from the tweater(s).

Acoustical treatments to the room can also be effective - usually best to have some hardish surfaces in the front part of the room / soft absorbing stuff in the listening area and diffusion behind the speakers.

REMEMBER:  Whether you like it or not - Your listening room accounts for about 30% of the Perceived Acoustic Performance of your system.

For people who have digital control of their systems, if a recording of female voices or violins has to much sibilance if you add a notch filter centered on 3 kHz, down 2-3 db you can eliminate it.