Hi Gringo and welcome,
For the record, the M80s on their own have useful output down to the 25-Hz region. By "useful," I mean that it's audible, but the output won't be nearly as powerful or linear as what you'd get from an Axiom EP350 v3, EP500 or EP600.
A well-designed and linear subwoofer will always provide deeper bass extension and greater output than virtually any floorstanding speaker.
I don't know the brand of your old sub, but it may have some useful output below its -3 dB frequency of 35 Hz, so you could always try it. If nothing else, it will help in getting improved bass distribution in different listening locations. Overall, I'd suggest you get a new really hi-performance sub. Your old sub could still be employed to even out bass distribution and counteract standing waves that produce cancellations and reinforcements at different frequencies in your room.
Regards,
Alan
