I've only used Invader once so my recollections of it are a bit hazy, but at least all the Engl heads I've used I thought were pretty easy amps to dial in, which exception of the Screamer because of the funky button configuration thingy, but after you get it down, it's pretty similar to other amps. But like with almost with any amp, start with channel volume and master fairly low, Hi-gain button engaged, channel 3, noise gate engaged, tone knobs at noon, presence up a bit, gain to taste and then adjust according to your guitar and cabinet.
And using emg's or other active pickups and a tubescreamer helps a lot too.
And using emg's or other active pickups and a tubescreamer helps a lot too.