Old thread resurrection - sorry! But when I read this I wanted to comment...
My band have a mixture of old and new gear, but essentially we use a basic mixer, active 12's on poles for FOH and a couple of floor wedges for monitoring. The rehearsal rooms we use have a nice PA available, and it's easy to sound good in there, but every gig was a frustrating quest to sound as good as during practice. We automatically assumed that our cobbled together PA was just a bit crap and were debating spending out on a shiny new one, but one evening we were sharing with another band and they made our PA sound amazing... their singer knew what he was doing and in a five minute sound check had it nailed.
We made the effort after that - read the manual(!), did a bit of YouTube research and spent some rehearsal time just working out how to get the sound in our heads out of our own equipment. Gig life is much better now and we can cope with most of the variables that real venues throw at you. So no answer to the vexed question of power, but a long winded way of agreeing that what you hear is not just about the PA... you also need to know how to use it effectively.