TEWhite1,
I believe both of those sites also have "plate amplifiers" that can be purchased to drive the subwoofer. They are basically amps that would mount right into the enclosure for the sub on the back side. I would suggest an amplifier capable of producing wattage equal to the sub's continuous power handling at the subwoofer's impedence (ohms).
B727guru,
Flat response would be when all frequencies are reproduced to the same level or within 3 db of each other. This is something that is dificult to "listen" for. A simple radio shack spl meter and a cd with sine waves on it will work quite well in setting up the subwoofer system. Depending upon you receiver, the crossover can be set on it (the receiver) or the sub. If you are using the receiver's crossover, turn the sub's crossover up to the highest frequency. If using the sub's crossover (depending on the receiver) just send the LFE output to the sub with the crossover off. The receiver's crossover typically will yield better results depending upon how expansive it is.
In terms of determining best placement for the sub, here are a few details.
1) Place the subwoofer at the listening position
2) Play fullrange pink noise through the sub only
3) Using the rat shack meter, slowly move down the wall (at the sub's position level, yes you may have to get on your hands and knees) the sub will be placed
4) Note the position with the highest amount of output. Also note other high points
5) After finding this position, test at various frequencies i.e. 20hz, 30hz, 40hz, 50hz, 60hz, up to 80 or so.
6) Note the levels (in db) at each frequency. use the placement that will give you no more that 3db of difference at each frequency. If you cannot get a position that allows this simply get as close as you can.