kopimon,
SignalScope will let you monitor signal levels in a couple different ways, so it could certainly help you to set your gains.
The headset input requires a low input voltage, between 10 and 15 mV peak. That may still work for you, if you have appropriate external circuitry between your equipment and the iPhone OS device.
Alternatively, you might want to check out the
Tunewear Stereo Sound Recorder for iPod, which will allow you to analyzer higher voltage signals via the dock connector audio input.
Ben