I didn't try this yet, but it MIGHT work. Most of the hiss is exagerated with high EQ, so try using an outboard eq, and keeping the internal settings at flat response. Since I got the Voice Prism, I doubt that I'll ever use the mic input again, except for my guitar, or in a nursing home type gig. It's a great convenience, but not such a great mic preamp. When time and tiny is important - the internal mic is going to have to do, but when the sound is a bigger issue, I will not use the "hiss box" anymore.
BTW - I keep the noise gate, AND the compressor OFF. They both color the tone too much, and I think the noise gate actually attracts attention to the hiss. It's another nice add-on, but not up to the quality of a studio gate. I'd rather see them leave out the sub-standard items and beef-up some other areas, like the flash ram section, or the flimsy keys.
Again, I have to repeat - It's still a greatr value for the money !