Research the "power handling" capacity of your professional amp and speakers. The power handling is listed in "root-mean-square" units, or "RMS." You can usually find the RMS information on a sticker or label on the amp and speakers, but in some cases you'll need to track down the user manual for detailed specs. If you're using the amp with a pre-built speaker stack, the capacity of the speakers and amp are already well-matched. If you're hooking an amp up to a concert hall's sound system, the RMS ratings may be off. Generally, you want the amp RMS rating to be twice that of the speakers.
Plug your instrument into the amp.
Turn your instrument's volume knob to the exact position you will use during the performance.
Turn on the professional amp and any attached speakers.
Turn the amp's output volume down to safe levels based on any RMS discrepancies. If the amp RMS rating is lower than 200 percent of the speaker RMS rating, then you don't need to worry about volume overloading the system. If the amp RMS is more than twice that of the speakers, you must adjust the volume based on the discrepency fraction. In other words, if the amp offers 400 RMS, and the speakers are rated at 100 RMS, you want to turn the amp down so it is only operating at half its peak volume. With a peak power output of 400 RMS, turning the knob halfway down puts your amp at 200 RMS, which is the maximum setting for 100 RMS speakers.
Position a friend or two in the audience. When you check a professional amp, it doesn't matter what is sounds like on stage. You need to make sure the audience can hear the amp well.
Play your instrument.
Adjust the volume directly on the amp or on an external mixing board until your professional audio amp is producing sound at a level that sounds good for the audience. As you adjust the volume, do not exceed twice the RMS of the speakers, or you risk overloading the system and causing speaker or amplifier damage.
Adjust the volume of other stage instruments or microphones to match the amp. Some mixing boards provide decibel meters for accurate sound checks. Without a decibel meter, you'll have to sync up the other instruments by ear. Basically, if the amp sounds louder than the microphone, turn up the microphone. If a different instrument is too loud when compared to your amp, turn down the other instrument's amp. Continue to adjust until all of the volume levels are even.