If your Bergs are close but come up a little short on the bigger, louder gigs, a pair of 12/6's will get you there and still have a little left to give if you need it, no need for the 15's, you can keep things small/compact with the 12's. They can do it because the woofers can take a good deal more power and give it back to cleanly, not "choked off", or sounding like they're running out of gas, assuming you have a fairly powerful amp. 300 watt @ 8ohm, 500 watt @ 4 ohm or bigger.
Of course, they will do it with a different tonality than your Bergs, so, a meet up/listen/comparison should be in order. The fEARful's will still have a "flavor", but they will be less flavored than the Berg's. The alpha mids used in the 12/6's are one of the nicer/smoother midranges of the line.