Hi!
From what I figure, the preamp alone should do just fine in powering the mics.
The A3 is actually nothing more than a battery box, unless U need the attenuation
and bass roll off features on it. The preamp U mentioned on here, doesn't seem
to work like a preamp, but rather like a battery box as well..
What kind of concert are you going to? Depending on the loudness and the type
of OKMs U have, U might have to go into mic-in instead of line-in.