If you get a plate amp with an adjustable 2nd order crossover, you should be able to match the mains perfectly.
Assuming a symmetrical 80hz, 2nd order hi/lowpass from the receiver, and a 2nd order acoustical highpass starting at 65hz on the mains from the sealed rolloff, you have a second order highpass at 80hz, that accelerates to an effective 4th order around 65hz and below.
To match this, you could set the subwoofer plate amp's lowpass crossover frequency (again assuming 2nd order) to 65hz, so you have a 4th order lowpass below 65hz. Then adjust your variable phase and you should be in business.
Also, unless you plan on building a super efficient sub, the extra channel on your receiver is not going to cut it IMO. I would be shocked if it actually put out its rated power, and it will likely not handle difficult loads well. It may work fine as a stopgap, but I wouldn't try to build a sub around that as the powersource.