Frequency range of an acoustic guitar goes down to about 80 or so hertz. Frequency range of a bass guitar goes down to about 40 or even below for a 5-string. That may not seem a lot of difference, but at these pitch ranges, that's over an octave!

A system designed to compliment an acoustic guitarist's rig will not be designed to handle the bass and kick and floor tom ranges that your arranger will produce, plain and simple.

At low volumes, and unhyped EQ curves, things should still be fairly even, but once you bring an amp into it's designed volume range (a 300 watt amp isn't designed to never go above 100!) you will definitely start to hear the designed frequency range coming into play...

PA speakers (other than those designed as the top part of a bi-amped system) are specifically designed to cover ALL frequency ranges fairly evenly, even at the top of it's intended volume curve. Things designed for instruments with lesser range needs, or the mid-hyped needs of band monitoring applications (combo amps in particular) are never going to remain as flat as they get louder...