You could get an active crossover and crossover over the source (say at 80Hz). Then send the stuff over 80Hz to the Marantz Stereo Amp and then to the main speakers. The sub 80Hz stuff would then goto a mono amplifier and then to the sub.
You could also get a line level output from the stereo amp and then to a filter and then to your sub, but this way means that your speakers are still trying to play the same low freq stuff that your sub is (a bit of a waste of power).
You could use a plate amp for your subwoofer, these sometimes have line outs, which you use to send the signal to your stereo amp and then to your main speakers. This is basically the active crossover and mono amp all in one...
Not to worry. You can buy a plate amp from a place like Parts Express which will allow for both low level and speaker output type connections. I have two subs running & I'm building a third. I like to use the speaker level inputs: they connect to the speaker outputs of your amp, then have outputs going to the speakers. These outputs are often 'shaped' by a first order filter at 120 Hz, which can help w/ blending the sub into a system.