As Eirk said double check the tap settings and connection to the amp's output terminal strip.
How loud are you trying to get in the lobby, those three little speakers will only do so much!!
Also there may not be a proper high pass filter on the line feeding that amp.
There should be a steep high pass filter of at least 100hz on those speakers.
Jamming too high of a level of music with a lot of low frequency would saturate the speaker's transformers and overload the amp.
Depending on how the outputs are configured on your mixer and stage box you may be able to do it in the mixer, does a separate mix out just feed the lobby?
The ideal place to apply a high pass would be in the DSP unit, you would need the software and most likely a password to get into it and a Bi Amp DSP is not something you just jump into, make a quick change and be done unless you have done a few of them.
At the very least you could turn the bass control on the amp down, maybe way down and see what that does for you.
