How do I smooth out violins?

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.