Our best oscilloscope for use with audio analysis is our ADC-216 scope. This has a 16-bit resolution and a sample rate of 333kS/s on one channel. It also has a 32k internal memory buffer when using one channel.
In order to EQ your sound system accurately, I would suggest using a high-quality microphone as these will have a better frequency response. The frequency response of the microphone is important because a poor response at either end would result in you increasing the frequencies too much in trying to balance the sound.
The microphone would ideally be a dynamic, cartioid microphone. You could use a capacitor microphone, but remember that these should not be subjected to very high volumes. The microphone would then need to be connected to the oscilloscope using a BNC connector.
One big thing to remember when setting up EQ is that the volume will make a difference to your perception of the frequency range. Lower frequencies are difficult to hear at low volumes but as you increase the overall volume they will seem louder in the mix. Therefore I recommend that you perform all EQ setting at the sort of volume you will be using when listening to the system.
If you have further questions then please do not hesitate to ask.