I certainly can have a basement actually, but a proper basement is $$$ - Just a subwoofer-sized hole in the ground is doable. Do you have any design I can take a look at?Ask your architect why you can't have the basement you need!
Then get out your ruler and graph paper and figure out where in the room woofers can and CANNOT go.
And - how do I know where to put that aforementioned hole? (or just right under seating position?)
Wait, did we stumble into diyforum from diyaudio forum?A 1% slope is going to look like a drunken builder's mistake.