Okay, John, things are becoming clearer. I assume that your pre-amp is designed just for stereo operation and doesn't have any internal crossover for a sub output. In that case regular speaker wire is used to connect the speaker terminals on the amp to the high-level speaker input terminals on the sub. The M2s can be connected with speaker wire directly to the same amp terminals connected to the sub, or ,if the sub has speaker level outputs for the main speakers, they can be used instead. If the sub outputs have a crossover to the mains for this second type of connection it should be described in your sub owner's manual. This would help to decide which type of connection to the M2s to use.
The 80hz lowest setting which you say the sub has is unusually high and you'd want to leave it at that lowest setting(assuming that it's relatively accurate; many aren't). The M2s would then be getting a full-range signal(unless the sub has a crossover on its output to them), but would be rolling off quite steeply below about 80hz on their own as the NRC measurements with the SoundStage review show. So, the 80hz sub crossover should blend fairly well with the natural M2 rolloff.
The sub location seems good, but the M2 separation seems a bit narrow. The rule of thumb is to separate them about the same distance as your distance from them.
