Synths, an SM57, and some cheaper condenser mics that can use batteries don't require phantom power.
You can't build a pre with any character that small, but something for drum overheads and such is certainly possible.
I love my Great River pre too. I first heard about it from a producer who used on a whole mix when mastering along with a Manley Slam. That is a bit much, but the sound that Great River gets from ringing a large transformer is superb for rock.
My mastering chain is a Manley Vari-Mu and Massive Passive then I switch in Apogee's soft limit function. I've tried peak limiting with a new Chandler EMI and a UA 1178, but they both mess up the sound. So now I use them for tracking different things. My Manley Gold mic has enough gain that I use it straight in to a compressor without a pre. So you can see that I also like a little analog compression prior to going digital.
What is a Kultube?
The modification can't be done practically. It was just an idea that I posted here as a warning.