The ISA One was already, in my opinion, the '˜boxed-preamp-to-get'. For all of my friends who were getting started in building their home studios, I often recommended 'If you only get one preamp, get an ISA One. It works on everything.' The ISA pres are known for being very clean sounding, and you can push them pretty hard without worrying about inducing noise. My only gripe was that if you wanted to record a stereo source, you had to get 2 boxes, (taking up valuable desk space since the ISA One was not a rack unit,) or you had to splurge and pick up a much larger 4-channel or 8-channel unit. The ISA Two fits very nicely in Focusrite's preamp lineup.
One of the features of the ISA two that I'm a huge fan of is the remarkable control the user has over the gain. The staging is done in a really interesting way. There is a selectable variable input impedance stage first. This allows you to closely match the general impedance to just about any microphone you can throw at it. This can be a real issue with certain mic and preamp combinations. Anyone who uses an RE20 can sympathize with me about the sheer amount of gain you have to throw at some classic microphones to get a saturated signal.
After picking the impedance that best matches your mic, you then get to activate a switch that puts you somewhere within the 2 ranges of gain (either 0 to 30 dB or 30 to 60 dB). Add that to the fact that the transformer starts you out with 20 dB and you've got a whopping 80 dB of gain to work with. That's enough to get a good signal out of just about any mic you can throw at it! (Don't literally throw mics at this thing, mics are expensive.)
You then have separate, dedicated gain and trim knobs. The gain control works in stepped values of 10 dB, and your trim control lets you add in another 20 dB depending on how hard you push it. This system of gain excites me as it truly lets you get quite precise with exactly how much gain you want to use to record a source. When recording something with subtle nuances like a grand piano or acoustic guitar this can be a feature that helps you dial in your sound to an extremely accurate point.
While we're on the subject of accuracy, I'm also a big fan of the calibrated level meters. I immediately calibrated the ISA Two's input meters to match the exact level that my DAW's meters were showing. I wasn't prepared for how confident and stress-free that would make me feel when recording after calibration. It makes me wish every piece of gear I had in my studio offered this option. My signals match up perfectly now, and with the precision '˜feeling' I get from using the discrete gain stages of the ISA Two, I feel very confident that I can dial in my sound quickly.
I was a big fan of the ISA One, so I wasn't surprised at all to discover that I am twice the fan of the ISA Two. The preamps sound great, the gain stage is extremely clean, and the calibrated meters put you at ease that you'll be saturating your signal without clipping.
Pros: Calibrated meters, precision gain stage, variable impedance for mic matching, single rack space, power button on the FRONT of the unit (thank you, Focusrite!)
Cons: I would have loved a S/PDIF output on this, or at least the option to add one via an expansion card