Never played a Powerball myself, but if you're after lower gain sounds you need to set the gain REALLY low on a Fireball. I have the gain at about 10 o'clock and it's getting into face melting territory.
Also, with the footswitch plugged in, there is no independent volume for the clean/gain channels. You need to have the gain set higher to balance the volume between the channels - either that or rig the footswitch so you can hit both buttons at the same time (the second switch controls master A/B)
The Fireball has a ton of low end and it is very tight.
In terms of cabs, I've never used an Engl cab. I have an Orange PPC412 which is fantastic, but I've also used it through a Marshall 1960A & B and a Mesa Recto oversize cab - it sounded great with all them.
Incidentally, I might be selling my Fireball......