This sort of crap is why I really wish people wouldn't just parrot catch phrases and clever sounding **** without knowing WTF they're doing.
"Box rise" is a misnomer. A woofer in an enclosure behaves as a rather complex electrical circuit with resistive, capacitive, and inductive components. Not only the electromagnetic component of the driver, but volume of air in the box, port(s), and even to some degree the acoustic properties of the room (vehicle in our case) the system is in all effect what the amp "sees" at any given frequency.
In short, at 40hz your amp may see a 20 ohm load, but at 25hz it may see only 1 ohm. The frequencies aren't so important, but the important bit is that at some point your amp very probably will see your nominal impedance. The only way to have a constant load at the amp would be to use a non-conventional speaker (like servo driven or perhaps planar transducer.... though I haven't really looked into planars enough to even say for sure there... but servo for sure).
So, if you plan to compete and your system is ONLY used to play one frequency and will never play anything else this is something you can try to play around with; if you plan to play music it's a red herring. Match your amp to your sub's nominal impedance and don't worry about it. Your quest to "get your money's worth" out of your amp will result in broken equipment long before you get dramatic audible gains.