If you return it, get the PF500 or 350 instead of the B2RE. B2RE was designed for people who don't really like the typical Ampeg tones. It's an OK amp but that's about it as far as I'm concerned.
However, the problem with your BA115 is likely some of the interior padding coming loose and/or needing more of it. I fixed my BA110 that had a similar rattle by stapling the loose sections of padding back on, then adding a sheet of 1" thick Nu-Foam to the area that wasn't padded.
Ampeg Portaflex Club #1