I am assuming you are happy with the B&W 602 S3 and LCR 60? If so I would look to fill the gap with another pair from the 600 series. Are you planning to use or move 602's to the rear as surrounds (or 5 channel-stereo)? Depending on the size of your budget and room, you might look at the 603 S3 or 604S3 for the fronts. The Martin Logan Montage is a nice speaker however it has a very different sound than the B&W in my opinion, you might find this distracting when listening to DD/DTS or 5 channel stereo. I have not heard the Magnepan MG12, but would presume the timbre match or lack there of may be distracting in multichannel. My 2 cents.
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Outlaw 990/7125; Parasound HCA 1500A; Samsung LN52B630; OPPO-BDP-83 Blueray
Belkin PureAV PF60; B&W 604S3 Mains; B&W LCR600 S3 Center; B&W 600S3 LS, RS, LB, RB
Outlaw LFM-1 Compact Subwoofer; Boston Voyager 3 (Outdoor zone 2); Harmony One