So you want to mess with phasing… I take it there aren’t any walls to interfere with the phasing, or they are sound deadened? I could imagine it being quite difficult.
QTX are not a great brand, that sub only goes to 40Hz. When looking for an amplifier, look at the frequency range, same thing for speakers. It might be better for you to use a car sub speakers/enclosures, they are pretty cheap, just watch out for the rubbish.
I used to use a 300W RMS Sub connected to a 500W RMS amp, it provided a nice punch to the chest. Put it in a room for 30-40 people and it was a lot less. You need a hell of a lot more power for bass than you do mids/tops.