No there isn’t any way to tell a Sonos amp to only play certain frequencies.  You can adjust treble and bass, but that’s not giving you the sort of control you’re looking for.  You could look at getting passive crossovers possibly, but that’s going to cut into your savings by not using a separate mono amp.  Technically, I don’t think you’d need a crossover for the sub, since that’s only going to play the low frequencies.  You’d only need it for the speakers connected to your ‘main’ amp.

There is no SONOS product known as “AMP Gen 3”. There is the current “AMP” an older “CONNECT:AMP”, and much older “ZP100” and “”ZP120”. Only the later version of CONNECT:AMP and the current AMP are compatible with S2.

In my opinion this is not a very practical scheme and there could be some sync issues it you are very sensitive to this sort of thing, but here is something to try: On the AMP with passive speakers connected, insert an open circuit RCA plug or unterninated cable. (Don’t use a shorting plug) This will allow you to adjust the crossover frequency. On the second AMP connect your subwoofer. Unfortunately, you will not be able to use the crossover and highs will continue through to the subwoofer.

There are some relatively inexpensive amplifiers that you could use for driving your subwoofer. This amplifier does not need to be exotic.

I may just go for the inexpensive mono amp to power the sub and use the sub out from the one Sonos Amp.  I can then put the other Sonos Amp to good use somewhere else in the house.  Thanks!

Oh, and the GEN 3 reference was for the Sonos Sub I was just reading up on, I just had it stuck in my head and mixed it up with the AMP.  Thanks!