Tried to use 2 STA-9 in bridged mono but the volume level was not equal on both. How to match volume level ??
Sure that is not normal, so something is either not working correctly, the system is not hooked up correctly, or a source or preamp issue is causing it.
To see if it is an amp issue, simply reverse the L&R channels and if it switches to the other amp the issue is not amp related.
If it does stay with one of the amps, then check all amp connections and bridging switches are set correctly.
And it goes without saying, but if you are using an RCA on one channel and an XLR on the other, the XLR will be louder and the RCA softer, but you already knew this.