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

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