Having a brief look at the diagram in the powerbox instructions, it looks to me like the amplifiers are supplied with regulated 6V plus or minus 0.1V. Therefore the highest voltage the amplifiers can get is 6.1V.

One third of this is 2.03V which is a bit further away than the 2.4 you mention.

Also, which type of chip is in the Powerbox Amp? Is it one that could cause concern?

I am not saying that I am correct but I am sure Powerbox would have thought of this before offering it as a solution.