+1 for the Octopre. IMO the pres are better than the Presonus'. I've recorded live and studio sessions with a Presonus Firepod and while I thought it yielded decent results, I feel that the Focusrite is superior.
In my personal studio I currently run a Focusrite LM56 with an Octopre light-piped in and it is by far the best sound I've gotten at home. I highly recommend it.
Regarding your other question: You'll want to run the output of your "nicer" preamp into the line ins of your interface. You may have to use an XLR/TRS cable to go into a line input instead of through a second preamp.