What about running the irivers gain at less than 0.
would there be any disadvantage to doing that?
NEVER do that. Absolutely horrible idea. Someone else can probably explain the reasons better, but:
1) You are taking a chance on distorting at a very loud show because if the gain must be set below 0 to keep the meters from hitting 0 db, there's a good chance your recording will be distorted. (If you did need to set the gain below 0 to keep the meters from going over, just leave the gain at 0 and attenuate with the ST-9100).
2) Besides, you get the most clean gain from the Church by setting the iriver at 0 or above. If you set the iriver at -10 dB, you'd only be effectively getting 10 dB of gain from the ST-9100 instead of 20 dB.