in the context of trying to get the acoustic guitars loud and loud in a mix of a fairly loud band, i can share with you my experience.
having spent nearly ten years playing in a fairly loud live band that has an acoustic gtr player, i feel your pain. in my band, the acoustic gtr is the basis for ALL of the songs and needs to be heard at all times. the line up includes drums(me), electric bass, electric gtr, keys (b3 and string pads) and 3 vocals. we play pretty loud and the acoustic has always been a problem until recently.
my brother is the acoustic gtr player. he owns several $2500+ instruments and had been playing them with one of those fishman blend set-ups that has individual controls for the piezo and internal mic with a TRS 1/4" that can blend the sound or split it into two outputs. he always split it. at the mixer, he would have our FOH guy create a blend to use in the house and send only the peizo to the monitors. it didn't really ever do the job all that well.
he recently did a bunch of research and talked to some notable national players and techs who all told him the same thing which is that if you are playing rock and trying make the instrument loud, you first need to play a cheaper and less "hi-fi" sounding gtr and second, you need commit to using only a piezo while using a soundhole cover.
he did it. bought two guitars that seemed to be popular amongst pro's and it works incredibly well. they cut through and feedback is gone.
here is what he uses live now:
Takamine EAN40C
CT-4B Preamp (included)
http://www.takamine.com/?fa=detail&mid=124&sid=62the guitars by themselves sound pretty terrible. he's now exclusively using the piezo through a radial passive DI to the PA and it's not bad at all. i asked him to buy that passive DI as many of the venues have phantom problems (yeah, none at all) and batteries die.
i was tracking him at a studio recently using those gtrs, just cutting scratch tracks for me to play along with. later, when i listened to them i was surpised how good it sounded. so good that if i HAD to make it work, i probably could. i won't be doing that, but if i was handed them to mix with no possiblitly of re-cutting them, i could certainly make it work.
EDIT: we do carry a countryman DI85 with us and use it when we can (battery not dead or board has working phantom). the radial does sound just fine though.