Well, the Studio One is based on a Neve preamp/EQ IIRC. It's a very straight forward preamp with controls that are very easy to understand... three band EQ with adjustable mid frequency, HPF and LPF, volume and gain. Even with the gain (red knob) all the way up, it's still quite clean. Each knob does exactly what you'd imagine it will do, and it's really hard to get a bad sound out of.
The Ictineo is also a very flexible tone shaper, but it's a little more esoteric in how it operates, and has no DI out (as compared to the Studio One). The Body knob is a mid boost/cut. Tone knob is a tilt EQ, and then you have all the mini switches which control a variety of midrange modifiers, an Ultra Low boost, and a level boost ('EUUUUUGE boost!!). Each knob covers a wide range of tones, and extreme settings can get quite... extreme.
Are they a little redundant? Maybe.
Do I miss either one when it's not turned on? Definitely.
The way I set my board, I usually start setting my basic tone with the Ictineo first. The settings in the picture above are fairly neutral actually, but it just kind of adds a nice smoothness and fullness to the sound. I kicked it on and off a couple times and while I can't quite put my finger on why/how, it makes a very pleasant difference to the overall sound just by being on and set "flat".
Then I use the Studio one to fine tune, set to the room, etc, and because it's so straight forward I will use it for any mid-set adjustments that might be needed as well.
Both/either have more than enough juice to drive a poweramp on their own (especially the Ictineo with the BOOST engaged!).
Hope that helps?