A Piezo can be quite good - I wouldn't worry about the rising Z using with a tube amp IF its a speaker without an xover on the woofer but has a 'Zoebel" conjugate network on that woofer - then the overall Z seen will be ~ that of the woofer plus conjugate in he xover region.

I might want a small resistor in series with the piezo in case the amp doesn't like capacitive loads at HF - what value of series do you suggest? (I think~2 ohm ok without much loss)

btw - you can try using a little speaker distribution xformer to get more spl out of the piezo - I think you then need a conjugate RL network to compensate for the capacitive nature - you might get ~105dB sensitivity that way - kinda a hassle when Foster horns are available cheap

Best Wishes!!!!!

Freddy