The Alpine is nearly identical dimensionally to the JL 600/4 mentioned above, though it is a little less than 1/2" taller. Given the clearance Zero23 mentions, the driver's side may work.

 

There's always the JL HD 900/5 ^^. Granted it's $$$$, it's same footprint as the 600/4. I woulda got the 900/5 if it was available at the time, but I didn't want to wait an extra few weeks, and I really didn't need 5 channel.

Alpine PDX's are good for the money too. JL has never ceased to amaze me with their quality amps though.

 

I have an Alpine mounted under my seat. I'm pretty sure we used a spacer or 2 to fit it. I hate it there and plan on moving it to the spare tire amp rack.

 

Above the fuel tank is fine and stealthy and just sized correctly for the PDX.

In my case, I have the PDX 1.500 and PDX 2.150 crammed above the fuel tank. It does fit, very snugly, and I haven't had issues regarding overheating.

I had to do a bit of cutting of the tray that is above the tank in order to get two of them to slide in, but if you are doing a single amp, you shouldn't need to do any significant modding. There would be lots of room around the amp for it to keep cool.

 

I have an Alpine PDX 4.150 mounted under the drivers seat. You can look up the specs and compare to the PDX-5. I used to have the measurements of the room under the seats, but can't remember where. There is not enough room under the passenger seat for any useful amp. I have not had any heat issues.

Mounting undrer the drivers seat, somewhat restricts how far forward the seat will move. I have the seat all the way back and only notice this when I am cleaning the interior.

 

I took a peek under the seat yesterday looks pretty tight it might fit but I'll never be able to access it either so its probably going over the fuel tank in the trunk or in the spare tire.