The battery box sends 9 or 12v (depending on the battery) to the mics. The higher voltage allows the mics greater headroom, therefore they can take a hotter signal without brickwalling compared to using plug-in-power from your recorder, which is something like 3v. If a preamp sends that same voltage, you would get the same results. if it doesn't you won't. Making a blanket statement "a preamp will definitely increase SPL handling as much as a battery box" isn't always true for all mics. I had a set of AT853's where the difference between 12v from the battery box and 8V from the preamp was just enough to avoid clipping at a Trey show. I deliberately ran battery box for 1 set, preamp for the other. no brickwalling with the battbox, slight brickwalling from the preamp. If you are living on that edge, then it matters.
The 4.7k mod is something completely different. To make a long story short, if you 2 mics are already terminated to a single miniplug, it's not something that can be switched on/off inside a preamp. That was originally intended for AT853's, I'm not sure if it even applies to BMC-12.
There were some early ST9100's that put out something like 3volts, then Chris made a change that makes it about 8volts, that was probably 3 or 4 years ago. I think it might have been when he came out with version 3, but I'm not positive. to check your preamp, find a typical cable with male mini-plug on both ends. Plug one end into "mic in" on the preamp. Turn on the preamp, check the voltage between tip and sleeve with a voltmeter. What you see is what you got.