This sound system would suit a rock-type band playing a small hall or pub. The band is a four piece, with Drums, Bass, Lead Guitar and Vocalist. The main requirement is to reinforce the vocal, as the drums and guitars may, in fact, be loud enough on their own to require minimal reinforcement. Often, the drum mics are reduced to snare, hi-hat and bass(kick) drum.
The most usual upgrade would be to add a couple of foldback speakers, say EV Sx200's run from an EV P600 Amplifier. This would give 2 channels of foldback, one for the vocals and one for the drummer, run from the 2 pre aux outputs of the Allen&Heath mixer.
The system could be upgraded by adding two more Sx200 Speakers to the existing two, and/or two more EV TL606 Sub Bass speakers. This would not require any more amplifiers.