You'll really need a separate output for the piezo if you don't use a preamp. For a flat response, the piezo needs to feed into an impedance of around 10 Megohms. A standard guitar amp has an impedance of 1 Megohm and the line input on a mixer of 10-47k ohms.

You'll get a sound from it if connected to a guitar amp, but it will be all midrangey and horrid. Maybe more acceptable for a resonator but the sound will still need a lot of EQ work to get it reasonable, which probably won't be similar to the EQ you'd want for the magnetic pickup.

You can use an external pre-amp designed for an acoustic guitar with a piezo - but again that's going to require a separate output.

Another option is an acoustic guitar amp with a 10Megohm input. I've just bought a Fishman Loudbox Mini acoustic amp (which is a very reasonable priced and good performing compact amp) and I found out after purchasing that it has a 10k instrument input, so can take piezos directly.

If you want to use the piezo rather than the magnetic sound, then you are better off using a full-range amp such as an acoustic guitar amp or an active PA speaker, as the limited frequency response of a guitar amp speaker will take off some of the top-end and clarity that the piezo gives.

So unless you want to use a separate amp or external pre-amp, then fitting an on-board pre-amp is really going to be the best sounding option when using a single output.