I had a similar set up as I transitioned from a receiver to separates. I used the receiver to power the center and surrounds and passed-through the L/R to an external amp. It worked fine.
However, because the Arcam and McCormack have different power and sonic signatures, you may find that the L/Rs don't integrate well with the rest of the system, ruining the surround effect. They also may not be "fast" enough during in scenes with heavy effects.
I'd suggest you uses a SPL meter to get the mains to the same levels as the rest of the system and watch a couple of your favorite movies. See if the L/R are noticeably different from the rest.
However, because the Arcam and McCormack have different power and sonic signatures, you may find that the L/Rs don't integrate well with the rest of the system, ruining the surround effect. They also may not be "fast" enough during in scenes with heavy effects.
I'd suggest you uses a SPL meter to get the mains to the same levels as the rest of the system and watch a couple of your favorite movies. See if the L/R are noticeably different from the rest.