Hello Philippe,
The REL subs are the best I have heard so far with the MG3.6R in my listening room. Mine integrates very well with the main speakers set at 34hz. The frequency range of the Stadium III is 18hz-100hz. The MG3.6R is rated 34hz-40khz.
The sub is to fill in the bass below what the panel bass drivers deliver, not replace them. The powered sub will not change the amp requirements of the main speakers. A big SS amp is my choice.