Hi, Tom! Welcome to the forum.
I've considered using a Sapphire XL as a center channel myself, so I'll be interested in hearing other folks' comments on this as well.
As for using shielded speakers, that's only an issue for CRT televisions. Plasmas, LCDs, and DLPs will be unaffected by the magnetic fields from the speaker. Mind, if you're using a Tivo or other hard drive-based PVR device, the speaker's magents could affect their performance.
Mike