I have been using Accusound contact strip mikes for over thirteen years now. I much prefer the sound that comes from the belly of the instrument to that picked up from the bridge. After all, I have spent a long time developing my acoustic sound, and that is what I want to reproduce. Of course, there will still be a difference; there's always an 'electric' feel about an amplified violin, but I think I get it as close as I can
You have to get a balance between the more shrill tendencies of the top notes and the darker tones of the bottom end. It helps to have a violin that gives you that range of sound quality in the first place! Violins tend to boom around the C# on the G string, so that's a frequency zone where you must take care about feedback. If you can attenuate around 270 Hz, you can control that source of trouble. Too harsh or nasal, and I find working on the 1kHz helps.
To protect the varnish of the violin belly, I use a strip of soft plastic, like the stuff they make non-adhesive window stickers from. It comes with the strip mic. Making sure that both fiddle and strip are really clean, I lay this across the instrument, just under the tailpiece, smoothing it across to expel air. I have stuck a small piece of this film to the black moulding on the end of the strip mic, and the whole construction can stay on the plastic strip by air pressure alone. Just make sure there are no air bubbles, by gently wiping down the strip-mic onto the clear plastic protector. That then is connected to a pre-amp on my belt, and then into my little 120w Gallien-Krueger 260 amp. For bigger halls, that then links into the PA, and I use the GK as a monitor.