I find myself in this situation again. Needing to power up an old acoustic to play amplified situations. I have put a Fishman bridge lick up in another old guild guitar. I run it through an L R Baggs gigpro on my belt. (I am switching that to my guitar strap to make changing gear easier).

I can balance the dynamics with the preamp and the board to get a solid acoustic sound. An I can easily hit the gain if I want to rat out the sound a little. Or I can always use a pedal.

But the rig is solid.

Don   

PS:

Guest, use a pre amp of some stripe on the harp. It warms up the sound and it allows for more usable volume.