Piezos place a capacitive load rather than resistive load, so they are voltage limited rather than amperage limited when figuring power capacity. The average piezo is very comfortable with about 25 volts in. When used in multiples wiring them parallel they all see the same voltage input. For example, 40 volts putting 200 watts into your woofers will probably toast a pair of piezos in parallel, both seeing 40 vols input. But if you wire them in series then the voltage is divided between them for 20 volts apiece, keeping them safe.For boxes that I don't plan on putting more than 75 watts or so average program into I use them paired in parallel for high SPL. With four or more I wire them as series/parallel pairs to gain both sensitivity and power capacity.
A vertical array of eight piezos gives better than 105dB sensitivity, is able to handle 100 volt signals, and can be built for as little as 16 bucks. That is a serious bang for the buck factor.