Viola Strings

I thought it might be time to change my viola strings when some of the low sections of the orchestra music were playing sounded a little too gravelly while trying to be quiet. I tried a different kind of string from Tomastik - the Superflexible variety which are only $60 for a set. They sound brighter than the Dominants, but I am still getting the gravelly sound on the C string in the piano sections. I am trying different bowing techniques, and perhaps I will hit upon one that will do the trick. These particular strings have stayed in tune pretty well since putting them on - one reason I like the a manmade core.