It seems to me, in addition to not having enough power out of the receiver to drive the sub to meet your needs and to really make a difference in your set-up, depending on placement, it also looks like you don't have the ability to control important items such as "phase" and crossover frequency to best blend with your M80's. These are vital requirements to get the most out of your system and are included in any half decent separate powered sub. I would submit that even though the M80's can go pretty low, a powered sub will allow one the flexibility to boost that bass in the 40-50 hz range, if you want to.
As was already mentioned, because of the greater demands placed on speakers and subs today because of substantially more advanced recording techniques for both movies AND music, even moderately priced subs go pretty deep and have internal amps with 300 watts and up in power. As "joema" already mentioned, however, even today's music recordings can vary a great deal in these areas of discussion. From my experience, Classical AND Jazz seem to have the most consistent quality since, for many years, these have been the primary choices for reviewers when they are testing the capabilities of equipment.
