Take all measurements under the seat first. LxWxH. From my experience: if the amp fits snug, don't get it.
Before you move forward with the purchase, think about the ease of installation too. Just because an amp can theoretically "fit" doesn't mean that you can install it correctly or easily. But that also depends on how stealthy or clean of an installation you are looking to get, what with all the connections and adjustments involved, it could get quite messy. For example: plan ahead and be prepared to make some slits in the carpet to run wires in/out of, keep your wiring alot longer than normal because you might have to connect them all before you slide the amp in place under the seat, etc, etc.
I'll see if I can get some pics up in the next few days of the Soundstream Picasso 1000D under the driver's seat and the 4.400 under the passenger's seat. They + the fused distribution block barely fit; and a clean, stealthy installation was a PITA, but it's all done now.