@spounder 

Shoulder rests and chin rests are tough to advise on because it is so relative to the person, but, I would think that if you are having a sore jaw, your setup is not right for you. It may be the setup is not right, or the setup is right, but you are jaw clenching, the best way I can explain it. Or, it could be both contributing to each other, but you should not have a sore jaw. 

I tried without a shoulder rest. It was comfortable, but it lacked stability for me, I have gone back to a choice of two. I am using a Kun or Bon Musica on both violas (15 1/2” and 15 3/4”). If I am having a good session, the Bon Musica is too confining, but it has so many adjustments you can make. 

Also, I have found, recently, that even though it is called a shoulder rest, for me, it is better as a chest rest. I now have it just high enough on my shoulder to catch the top of the slight curve in the Kun on the shoulder and the rest of the shoulder rest is against my chest. This is where a shoulder rest with optional longer legs comes in handy. They can raise the instrument up from the chest. You can also add a padding to raise it. You still have to have the shoulder end raised to the proper height for your neck space.

This may not work for you, If you think that you are “jaw clenching” because the viola is not not steady, or in your mind it isn’t, I would recommend you try the Bon Musica. You might switch back to a less restrictive one later, but the stability it gives, may help you relax your jaw grip, until you are used to it.

If you think you are having to lower you head to the viola, you may want to try a higher chinrest, rather than raise the viola. But, that is preference, too. If you are dropping the head. I would think you need a higher chinrest (prefereed by me for me)  or a higher shoulder rest at the shoulder end. Just don’t go so high you are playing with the head raised or tilted backwards. 

You also could have a chinrest with too aggressive of a lip. I can’t use an aggressive lip chinrest. It digs into the underside of my jaw too much. 

Look in the mirror while you are holding it as if playing it and check your posture and see if you can see and feel what you think is causing the jaw clenching or whatever issue your setup is doing. Pay attention to how neck is bending, the jaw feels against the chinrest, etc. See if you can see what you may be subconsciously doing to balance it.  

Just so suggestions from what I have found,

I am happy it os working for you. Don’t be surprised, if after you are more secure with your viola. you want to try a different setup, it happens. I still use my Bon Musica viola and violin shoulder rests, and switch between the Kunn an Bon Musica with the viola, and Fiddlerman and Bon Musica with my violin. It depends on how I am feeling.

Today, it was the Bon Musica.