for music, i suggest an integrated amplifier (for wharfedale speakers-pioneer integrated amps will do,puwede rin NAD amp.) since it is really built for music,not an AV receiver. your Onkyo 636 might not provide the amp juice you need for your speakers considering your type of music: rock,metal and contemporary type of music with double bass sounds.
Onkyo AV receivers are excellent in movies,but average only on music (especially your music preference) because of the reality today that manufacturers tend to pack unnecessary features and features you might even need at all (featuritis) just to compete in the mass market.
your room is considered small (as per standards of Wharfedale which provides that small room=20 sq.meters and below.)
even if your room is considered small, 12"sub is not overkill for your type of music with double bass sounds, it might even be the most appropriate under the circumstances which can generate low,deep subsonic bass for the music which you crave for.but it will not be appropriate for classical,jazz,instrumental, and voices type of music. puwede mo naman anytime hinaan ang volume pag medyo malakas pero pag kulang sa bass output, mas mahirap kasi your option would be to buy another sub.