I don't know much about violin, but I do know a thing or two about pickups :). Piezoelectric pickups are nothing like a standard electric guitar pickup in that no magnetic field is induced while you are playing--which makes them suitable for applications such as violin and classical guitar where no metal is involved. Due to their construction they have much higher output impedance than an average passive pickup and consequently use an output buffer to properly drive the line. Unfortunately, this combination of features will never result in a completely natural violin sound. Output buffers tend to be very simple to make, and they almost always color the tone of your instrument because they are designed to boost some frequencies and attenuate others in the name of controlling clipping. There are some that have been manufactured to have a flatter frequency response (David Barber's B-Buff comes to mind) but I haven't seen any of those incorporated into a piezoelectric pickup, and they are large and bulky due to the higher quality components required to build them than a standard JFET/OpAmp buffer.
So, with this in mind were I you I would simply continue experimenting until I found a pickup that I was completely happy with. A piezo will never sound as lush or natural as a nice ribbon microphone or even a lower end condenser due to the reasons discussed above, but when looking for a good piezoelectric pickup consider the following features:
- Controllable EQ. If you can control which frequencies are attenuated/boosted from the pickup then you'll have better success dialing in a sound you like. You can also achieve this by running the line from the Piezo into a pedal such as the L.R. Baggs paracoustic DI and controlling it from there.
- Higher quality or transparent buffer/pre-amplifier. This will control the tone coloring I discussed earlier. It will never be perfect, but you can get very close with nicer components.
All things said the best tone from your violin will come from a nice microphone, but like all situations similar to this it's the musicians plight to compromise and find the best trade off of practical application to tone. Don't be a perfectionist unless you need to be. Studio musicians and people who play in symphonies will almost always choose a ribbon or high quality condenser microphone over a pickup, but touring fiddle players with a country band might live with a piezo and EQ it to their liking. Find the right compromise for your application.