When your speakers are 8Ohm you still get 10W.
The signal is coded as voltage. Higher voltage
swing requires more current (use Ohm's law).
If your amp can deliver the current it is fine,
if not the signal get distorted.
If you use 4Ohm speakers you should get more power
with 2 zens in parallel (4Ohm speakers require twice
as much current for the same voltage). One zen
could get problems with it but with two you should be
If you use 2 zens with 8ohm speakers you
still get something in return, namely,
I do not know if the Zen runs on a a + and - voltage rail. If it does: in stead of running them in parallel you could opt for suppying one part with the 'opposite' input. Attach the speaker to both "+" terminals and you acquire part of the 'super-symmetry' which is a feature of the X- series...
That increases the voltage-swing over the speaker.