I have heard the bigger the electronics cavity the better. I am thinking of doing the same to an older Ibanez btb I just threw a set of Nordstrand big singles in to. The control cavity on that bass is a 5 knob set up with the batteries in a separate cavity, so it should have enough space to fit in the preamp and components.
Your Thumb BO, probably has the three control set up, with the battery inside the cavity. That means you you may be in for some routing work, given that you'll be adding probably 5 control knobs, a different board, and still need room for a battery (possibly two).
I am interested to see what others say though. Judging by mike's attention to detail, and the support he has gained from builders and players alike... They seem to be a pretty stellar preamp. I look forward to dropping one in someday soon!
BTB Club 120.