I haven't played a Powerball, but from the soundclips I've heard they're a bit different. Also, the Fireball is 60 watts (although it's far and away the loudest 60 watts I've heard, I'd guess it was 100 if I didn't know better), whereas the Powerball is 100.
If you're interested, go listen to some of rocksolidamps' soundclips, and read some reviews at
www.harmonycentral.com