Are Anthem amps bridgeable?

No. Besides the issue of adding cost for something that most people will never use, an amp loses its ability to drive lower impedances when in bridged mode. Bridging is useful for PA systems that get reconfigured from venue to venue, but for the home it makes more sense to use the right amp from the start. If more power needs to be added afterwards, and the speakers happen to be bi-ampable, then bi-amping is a much better solution.