If your camera has different focus points, take the time to adjust the focus point to the area of the photo you want in focus. [Please read your manual if you are unsure how to change the focus points.] On my Nikon D200 and D50 the adjustments are made on the back of the camera. The D200 has a 'lock' feature to ensure the focus point doesn't change when the camera is brought up to the eye.
When you are shooting wide open [aperture to the smallest number e.g. f/1.8] the area in focus will be very small. Hence, if you do not change the focus point and leave it in the middle nothing of value will be in focus. In this case I focused and exposed on the face of the closest guitarist [singer] and used spot metering combined with manual exposure. If I would have used aperture priority, when the light moved over the second guitarist from the front guitarist my camera would have changed the shutter speed to match the change in light. I have found in low light photography manual exposure is the setting I use more than aperture priority. This might not work for you but my suggestion is to experiment using manual exposure combined with spot metering and make note of the results.
Experiment, practice, and have fun!
More to follow...