include any sort of removable endplates on the amp? There is not a lot of room to spare with my amp. I saved over two inches on my amp by removing the endplates. I looked around A LOT before buying my Rockford Fosgate 551X 70W X 4-channel amplifier. A large part of my decision was the relatively small size of the amp. You could fit a larger amp but might lose some adjustability of the seat sliding back and forth.
The only way you will know for sure is a trial fit. Just make sure you don't scuff up the amp in the process by dragging the seat over it, which could make returning it problematic.