How do I smooth out violins?

Interesting.
I play the violin everyday. So does my son.
Reference real sound, and proceed from there.
Massed violins are hard to reproduce. The information is very complex and the ear is very sensitive.
I believe that there is no cutting corners for accurate reproduction. You can mask an error in the playback chain and make it more tolerable by adding more distortion but the original distortion and the added distortion can't be corrected once there. Stacking the distortions is like trying to correct for too much salt by adding more oregano to your soup.
Unfortunately, distortion creeps in everywhere. It must be minimized.
There is no free lunch. Cables ring, digital jitters, amps feedback, speakers make a mess, etc.
The only way I have improved playback is by minimizing distortion, not masking it.
It is not an art. It is science. Go from there.
This lesson repeats itself over and over again.
We are not monkeys typing Shakespeare, we have reason.
There are many ways to mess it up, only one way to get it right. Limit distortion!
-Mike

As I'd mentioned, I was experiencing the same issues with high massed strings in the 1kHz-3kHz region, with my two systems, which have very different component classes and room environments. The only thing they shared is a redbook D source and now passive preamps.

I mentioned getting an Empirical Audio Synchro Mesh reclocker (SPDIF in) and am very pleased with this excellent product, which helped. What has also been interesting is that I hooked up a cheap ($25) linear power supply to the SynchroMesh, taking the place of a switching supply wallwart, and that has really also been positive. Based on that experience, I am currently working on the theory that PART of the problem with higher frequency massed strings is the drawbacks of switched power supplies (SMPS). They s***. I'm now looking at a variety of power improvement products, testing the hypothesis that ac problems are a big issue. In the past, I had noticed positive improvements in this area from shielded power cables, by the way.

So my current assessment of the high strings issue in digital playback is that it has a complex set of origins:

1) Faulty recordings;
2) Redbook CD is either the cause or is particularly sensitive in this sound area to system problems.
3) Jitter - no doubt that this is a contributing factor;
4) Dirty power. I'm continuing to work on this. New fuses are ordered and I'm going to get audiophile receptacles. I am almost convinced this is another major contributing factor.

Re-reading this thread, one thing that struck me about Bojack's system is his patently inferior pre-amp. My personal suggestion for you Bojack is to try out a passive preamp. Very good units (Goldpoint, the Lightspeed Attenuator, etc) are well within your budget.

I'll update this as I learn more about the ac issues.

Bo...violin (string) sounds are actually "sawtoothe" waves.....extremely difficult to reproduce electronically. Its a rising pulse followed by a sharp drop....you need fast equipment to cope. ..here's some info that might be interesting. The hair on a bow has microscopic "thorns" along its length. When the performer draws the bow on the string, these thorns actually pluck the string, but then the pressure of the string on the bow immediately stops the string from vibrating....thus...the sawtoothe waveform.

Stringreen - That's actually a very interesting post and it taught me something, BUT the problem in my system is only apparent in orchestral music. Solo violin in that higher frequency range actually sounds exceptionally good in that range, massed violins exceptionally bad.

I see the weak points in your system as the Marantz CD player and the NAD preamp/tuner. As a Spendor SP1/2E user, let me attest to their ability to render classical music very well indeed. By the way, I became even happier with the Spendors after adding a good subwoofer to supplement the low end. Classical music is much enhanced with Spendors combined with a sub (same with other music). Bottom line: No free lunch. Your loudspeakers deserve much better electronics.

You know, distortion affects all signals passing through your system.
The nasties you describe are always present, even on the solo violin you think sounds good.
On some of the signals it is more obvious, that's all.
I read a post where someone suggested that the more diverse your system sounds with different source material the more likely that it is neutral.
I think that he is on to something.
All of the distortions in a given system color the sound. Many together make mush.
I have found that the closer I get toward neutrality (non-distorted sound), the larger difference small changes make.
It is like peeling an onion. I keep chipping away and all of a sudden things really start to come into focus. At first it's a leap of faith and then the closer I get the clearer it becomes.
An analogy is color mixing. Too many and you get muddy brown. Narrow the blend and it becomes clearer and brighter. Distortions are like those colors. The more of them you have the muddier the situation becomes. Peel them away one at a time and only after most are tamed does the picture become clear.
-Mike

I notice that in this and related violin forums, nobody mentions the complexities of the human ear. An early very sophisticated attempt to explain vagaries of massed-violins tone is by Mark Dolson in the early 1980s. This paper is not directly useful to improving the sound of your hifi set, but gives you some ideas.  eg, I have many CD on which massed violins sound fine; but I have bought 5 CD of Rachmaninoff's Piano Concerto #2, and the violins are bad on all.  Does Rachy score the 1st and 2nd violins in some peculiar way that creates "Dolson Effects"? I can hear massed-violin problems even at high-quality live concerts, the Vienna Philharmonic, eg. Wall & ceiling reflections can damage massed-violins tone—also, like Dolson, published in acoustics journals.  I suggest that female-chorale distortions arise similarly.  Of course, it is highly likely that the situation gets worse via the processing and playback chain.     

I used to have a McIntosh C50 solid state preamp and swapped it out for a C2500 tube preamp, and that change not only smoothed out violins, acoustic sounds (voices, pianos, guitars, etc.) also seemed to sound more "natural" to me.  Then I swapped out the stock tubes for Gold Lions and that further improved the sound.

Get a better source and add ac filter chokes to your preamp and amp!

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.  

Violin sound is actually a square wave....very difficult to reproduce ....you need good equipment to do it right.