That Amp personally looks a little too high and you will have issues unless you jack up the rear of the seat. I've got a Pioneer GM-D510M under the passenger seat and the seat 'just' touches the top of the amp, enough to only take a coat of the paint off. I guess if you cut the carpet out you'd get a little more depth to it, but I'm not a fan of chopping the carpet up in the event of selling/changing things around.
 
http://www.pioneerelectronics.com/PUSA/Car/Amplifiers/GM+Series/GM-D510M
 
Good luck. The link above will give you the height tolerance you can use thou on what will fit.

Heres what i have done, i didnt recomend this earlier cause you said you wanted a stealth install, this can be seen if looked from the side window. i have one on each side and have cut into the carpet as can be seen in the pics. if you store you targa shades there it will be hidden.
i did it this way so i can store my shades as well


 

I have looked into under seat amp install but max clearance is only bout 47mm making it hard to find a DECENT quality amp in such a low profile as most are bout 52mm an that's with removing the underlay.
Boot install would be good option just be careful running cables as u may induce noise.. Could poss put cables in flex metal conduit