Unity gain is the level setting on a recording device where you aren't adding gain or attenuating the signal into the device. If you have the original R-09 (NOT the R-09HR), I believe unity gain level is 13. I've read, however, that you can go down to 8 or 9 before the R-09 brickwalls (read: distorts the incoming signal).
I don't remember U2 being terribly loud and, assuming it's an outdoor stadium, the sound probably won't be too intense. Since the gain on the SP pre is only switchable as opposed to having an adjustable pot/slider, I'd suggest unity + the 29dB setting and see how that works for you during the opening band and adjust accordingly; ideally, you'd like the pre to do most of the heavy lifting as, in theory, the SP pre should be quieter than the R-09's pre. Keep in mind that U2 will probably be mixed a little louder than the opener.