sub integration with sealed mains

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I am building sealed JH3 3-way mains which have an F3 of 65 and a F10 of 37. I think they will need a sub. This is for music only. My reciever has a neat feature where it can drive a passive sub with 70 watts but the crossover is fixed at 80hz. Do you think I would have a good integration with the mains? or should I plan on buying a plate amp that has an adustable crossover?
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.
Morbo is assuming that the amp contains a crossover as opposed to a filter. He makes very good comments if you do have a crossover.

Most amps I know of though use filters, where your main speakers will not be affected and run full range, so all you need is a full range output to feed your plate amp and use the plate amp filter. The filter frequency will need to be determined by ear as it will depend on placement of both mains and sub.
not sure what you mean richie, I am assuming that the receiver has symmetrical 2nd order slopes on the low and hipass, and the sub has a 2nd order adjustable lowpass filter, which seems to be the most common type on plate amps. I am not assuming the plate amp has an adjustable hipass section if that's what you mean by crossover vs. filter.

Then you run the sub out (already lowpassed at 80hz 2nd order) to the sub amp's line-in. Then set the plate amp lowpass at 65hz.
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