I have been experimenting with pickups. I found that a Schertler Basik contact pickup (electrostatic, it's a little condenser mic) gets great arco and midrange response and blends well with the internal pickups. There's nothing wrong with what's installed by default, it's a matter of personal taste. It's a less focused sound, as it picks up vibrations through the body.
It sounds best when it is pressed against the body of the instrument, though. Not super-hard, just a little. It makes sense as the range of motion in a solid body isn't likely to be as much as a thin top on a hollow instrument. Another well-known electric cello exhibits the same treatment: the pickup elements are held to the bottom of the bridge by a brass plate, under tension.
My local luthier is helping me with a mod, to mount this pickup to the back of the instrument under controllable pressure (adjustable with screws), and we're going to wire both sets of pickups into a TRS jack. A K&K Trinity preamp buffers the piezos and sends 9 volt phantom power to the Schertler. The Schertler Basik is rated for 10V, so likely no problem. The K&K Trinity is designed for a different microphone, but I've gotten good behavior out of my Schertler plugging into the K&K.
This arrangement allows me to blend both pickups into a single output, or to send them to two separate outputs, and to control the level of each pickup separately regardless of whether they're blended or not. I lose realtime control over bass/treble/mid, but honestly I don't touch those so often, and that kind of EQ belongs on my pedalboard where I can switch it with my feet.
Pickups and tone are a highly personal thing, I stress and re-stress that nothing's _wrong_ with it the way it comes. But I got into this knowing that I wanted to experiment, and it's an excellent platform for such experimentation.
This mod is easily reversible and preserves the original bridge and pickups; only an external strip of wood gets attached to the instrument.
I'll have photos when my luthier is done with it. I estimate 1-3 weeks depending on his workload.