Lower tension giving more power is sort of a myth. Studies have been shown to conclude they don't necessarily provide the power one thinks they are getting in terms of "pace equating power".
For example, if you strung your frame at 60lbs and hit a 100 mph serve, then if we followed the myth, surely stringing it at 50 or 40 lbs one would be able to hit 110, or 120 mph serve for example.
Fact is, you would hit about a 101 mph serve. So yes, the drop in tension gave you one more mph, but human beings aren't sensitive enough to detect a 1 mph increase.
What actually happens when dropping tension, and where this myth began is that when you drop tension, using the same swing path, the ball defelcting off the string bed leaves at a higher trajectory, resulting in the ball landing deeper. The ball landing deeper is perceived as hitting with more "power".