Re: battery box or pre, I would get a small battery box from Sound Pros or a more expensive but more versatile 9100 preamp from Church Audio.
Or a small battery box from Church audio. I love the ST-9100 if you want to spend the money.
It is possible though, that you could use another set of mics as well in addition to the battery box. SP's makes very good stuff, but I checked the SP-2's specs at the SP's website and they only go to up to a 105 dB SPL (and it doesn't say that they go higher with a battery box). They probably sound tremendous for ambient stuff, which is what they are designed for, but may well distort at a loud rock show even with a battery box.
If you do want to try another set of mics, I would get CA-14 cards or omnis from Chris Church plus one of his tiny battery boxes.
There are many other good options also, but if you are on a budget I feel the CA's give you the most bang for the buck.