"Wally" wrote
It occurs to me that, since an isobaric sub has two drivers, then there
are
two options for connection, series and parallel, which results in two
options for impedance - 4 and 16 ohms.
More like 4ish(f)+/-jX(f) or 16ish(f)+/-jX(f) ohms.

In the interest of giving the amplifier the best chance of maintaining
tight
control of the drivers, is one impedance preferable to the other? For
example, what would happen if I put a 16 ohm speaker on an amp that
expected
8 ohms?
4ish+/-jX ohms may blow your amp or at least demand high current from it.
16ish+/-jX ohms will have lower damping as each driver is driven from
8ish+/-jX ohms.
Best option would be a separate power amp for each driver. Make sure out of
phase.
Second best option is in parallel but note comment above.

The amplifier will likely be solid state, 50-100W per channel, maybe with
a
low-pass filter at the input (because that seems like a good place to put
it - all of the amp's work is going into the wanted frequencies).
Yes, this will reduce IMD too.

HTHAB,
Pete