2.5A should be enough; the amplifiers on the bonnet only take about 5mA when not doing anything, so shouldn't shut everything down. At worst, if it was on edge, I would expect that extra load to turn the red LED off but otherwise not do too much. Certainly not totally shut the Pi down.
So you would seem to have a problem that's drawing far more than a few milliamps.
My first thought is that you have a short circuit between 5v and ground. (According to the pdf https://cdn-learn.adafruit.com/download ... rry-pi.pdf
page 11, the bonnet uses both 5v and 3v3, but I've searched the schematic, and can't find any connection to 3v3 other than the group of holes towards the left of the board)
Try looking at the soldering around the first few pins of the header, where 5v and ground come into the board.
If you have a meter, you could try measuring the resistance between 5v and ground. If you haven't got a meter, think about buying one!