Yea - I agree with the reciever crowd as well because without that it will be hard to use that sub. If you wanted just stereo, the best would be to get a pre-amp with some kind of bass management (you need that to cross over your M2i's and Sub) and a power amp or 2 (mono or stereo).

In my opinion that HTR-5550 will have more than enough power especially considering you won't be using it for bass, as you will use its crossover.