I'm pleased to hear that you're very happy with your new M80's. This is not a surprise to us, which is good.
While I understand that the bass response of the M80s is very good, particularly for most music, it's when watching movies that you would get the most benefit from a good subwoofer. Movies such as Avatar include very low frequency sounds that even the M80s cannot reproduce, and it's those sounds that you feel more than you hear that really pull you into the experience of what's happening on screen.
Adding a subwoofer is highly recommended for this reason. As well, the other speakers will likely sound better to you when you direct the lower musical frequencies to a subwoofer. This essentially "frees" the other speakers from trying to produce those lower sounds, which is difficult for some speakers, and allows them to handle only what they can.
Adding a sub has many benefits for any system.
Just know that those lower frequencies tend to travel further, so make sure that your neighbors are invited to your party once in a while.
