You need to take care two things, the first is impedance, your headphone amp is probably fine to drive 30Ω but if your load is 8Ω coil it probably won't like it, a small power amp is probably a good start, the power stage of a small guitar combo would be my first try probably, I don't know what you have as element. Same for the preamps, piezoelectric elements will have many pF output impedance, mic preamps are optimized for many ohms, and your pF will be many mega ohms at LF, so you have a problem there, probably a get input would be a better match for it, with properly high input impedance.
The second one is the usual power amps used in plate verbs have a particular filter, for compensating the losses. I attach a file for a plate reverb with some schematics, you can avoid from there the line receiver at the power amp and the line driver at the preamp, but I recommend to use a power amp as stated there, with a similar filter and a high impedance non inverting amplifier for the piezo. You only need 1 NE5532 and 1 LM1875 (20W TO220 amplifier) for the driver and one LF356 (opamp with 1G input resistance) for the preamp. You shouldn't have problems to find those around, then you can add later a line receiver and line driver so you can run longer wires balanced without more noise or losses, since you usually want to have your plate reverb away from the mixing room, specially if you are mixing loud. But not need to do it as a first approach since you will be working with your verb close because you are tweaking it.
The power amp you probably want to have it close to you, since you have tone and level control, the preamp will be probably better close to the piezo pickups, to avoid picking up noise in the way to the preamp. You will find a spot on the level where it works good and leave it there, and control the verb level from your power amp, or have a separate output level in a stage after the preamp, so you have the return control closer to you.
JS
