Having a little problem understanding the basic premise. Your "folk-rock" band has a PA; are you planning to add the Bose on top of it? If you can't hear yourself in the mix, tweaking the monitor set-up would seem to be the answer. Perhaps an in-ear monitor with a separate mix, if that's do-able.
I have heard solo acoustic acts using the Bose set-up a couple times. It seemed to me that adding the "optional" sub-woofer was a necessity, since even with Bose's super-sophisticated EQ set-up, the small speakers in the array don't carry bass freqencies all that well.
Concur with Tony's evaluation; sound experts to whom I've talked, say that Bose's approach is to use EQ wizardry to overcome the limitations of a system that would sound pretty poor without their specific circuitry. And the system ain't cheap.
Just my 2¢, though...
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