It sounds as if your string section is suffering from the "lack of up front" presence often mentioned about Ty2 voices.
This is being addressed here: http://www.yamahapkowner.com/forum/index.php?topic=7538.0;topicseen
In part 2 in that thread is a procedure for beefing up the Ty2 voices which "should" (fingers crossed) be do-able on the S900 as well.
Suggest you carry that out, then audition the Sweet Violin. There's a huge difference which hopefully will be what you want to hear, not only in volume but nuances also. Check out other voices too to be convinced. Don't take my word for it.
Note: The procedure was put up yesterday mainly based on Ty2, so have yet to hear confirmation that it WILL work as described with the S900.
If it can't be done using a template from the supplied registration as described (ie. the S900 doesn't have the Function to freeze voice settings) it will still be possible to do, albeit more slowly by editing the individual voice(s) using the suggested settings, assuming it has the usual Yamaha onboard editor.
John