Since you have the Oppo, you could go either way. Depending on what Oppo BD you have, you could even go with just a pair of powered speakers or a two channel amp. You won't get any room correction though if that is important to you. Personally, I enjoy stereo sources processed into multi-channel just as much as in their native format, but I'm not an audio purist either. A stereo source processed over 5, 7, 9, or 11 speakers sounds great IF the room correction is top notch and the room is treated properly.
You can save a bundle on the Marantz ( refurbished ) if Accessories4less ships to Canada.
HD-DVD = 94
Blu-Ray = 120 ( 24 Warner red2blu )