I've seen two data points... "6 point something" volts (like 6.5 maybe?) and that worked OK... sounded great. Another case of "3 point something" volts, and that one brickwalled... pretty distorted. This was an outdoor moe show... reasonably loud, but certainly not like indoor death metal.
I don't know if anyone has much data as to how long typical 9V alkaline batteries will last in that rig before they start to drop off. For starters, I'd suggest putting in fresh batteries whenever you are heading to an important show, and save the old ones in your Ex-wife's smoke alarm.
