I own the Monitor Audio RS6's and I am currently driving them with my Onkyo A-9555. The sound is detailed, lively and musical. I had been using a Krell S-300i with the RS6's. The Krell is going for service so I substituted the Onkyo. I am enjoying that combo so much I will not be pacing the floor waiting for the Krell's return.
The A-9555 may have been discontinued but for the money it would be worth a search for one or a gently used one. I paid $500 for the Onkyo and $2,500 for the Krell. I would rate the Onkyo the better value.