>> Could you recommend an amp to drive the Pi bass units?

I like Crown amps, lots of clean power. But there's a bunch of other amps that are suitable too.

>> Don't know the efficiency of any of these or how
>> they would cross over.

Obviously, this is application specific. But you could do it actively or passively without much difficulty.

>> How low would the 10 Pi option go down to?

A 2226-based ten π horn is strong down to 40Hz.

>> Where can I listen to one?

How far are you from Tulsa?

>> How much bigger to get down to 20 Hz?

Huge. I'd say horns that are useful to 20Hz would be for permanent installation only. You can make a reasonably sized reflex cabinet reach 20Hz, or you could make a transmission line, possibly a TL pseudo-horn. But a true basshorn that reaches 20Hz is a massive device.