It won't do both FOH and your amp, at least not that I've heard. To me it downgrades the sound in the house if you split the signal. And while it can be used in certain power amps as a preamp, it's really not its intended use and doesn't work in every power amp. REDDI's one of those things that does only one thing but it does it better than anything else out there, or at least that's what I believe, so I don't worry so much about what it doesn't do. OTOH, A-Designs does make some preamps and they're pretty nice as well.
