Hi PandAura - in general terms most units providing a line level or balanced output as appropriate should be suitable. Weather its a DI (direct inject) box, an active crossover or just the output of a different mixer the amp will just do what it is designed to do - amplify it.
The output problems arising are generally dependant on the input type and inpedance of the amplifier itself - its actually the one thing that AA can't control however most units deliver over quite a wide tollerance band where as the VMS4 is right at the lower end of this band - hence the problems. This is the first time that I can remember I have come across a mixer of of this size that runs from 5V supply. They are usually around 7.5-9v although I understand why 5v will become popular as this simplifies design issues relating to USB connections etc.
The real point is - this unit was deigned not to need external assistance. Although the additonal cost of such an item is undesirable in real business terms it is small - its the inconvienince of more kit to set up every time.
On a sparate note I would point out that I have a variety of other AA equipment, particularly lighting effects, that are well made, rugged, very reliable and would thouroughly recommend. I am not saying I would never buy any of their products in the future as appart from the VMS4 I am impressed with their kit. They just need to hold their hands up, admit they have an issue (which they have so far) and ultimately get it sorted promptly - which seems to be their downfall.