I assume you are just trying to find a suitable level to "practice along to" as distinct from recording the final piano-plus-violin?

What I find handy (both for practice and, essential, for me with my setup if recording a solo against an existing music or click track - I don't use a pickup on the acoustic, rather I use a studio mic - just gotta remember not to cough, curse, or stamp my feet!!!) is to use headphones.  

I play the backing or click track through the headset, with the left earpiece in / 'phone over the ear, but the right ear remaining uncovered.  The 'phones I use have "reasonable" deadening to external "noise" ( i.e my playing, haha ) so my left ear doesn't get totally deafened by its proximity to the violin and can cleanly pick out the music/click track - while my right is catching more of the violin and my "IF" ( intonation failure) bits.... LOL

It's not perfect, but it helps for me, so I'm not "fighting" against room echoes, position/distance etc from the sound source - and the arrangement is reasonably "repeatable" and "controllable"

Bill