Seriously, Steve is making the point that patching through the powerhead as a frontend could make bi-amping a bit of a chore. That's what I think he means, anyhow.
I have no idea what sub inserts are, never heard of them. As far as my mixer goes, I don't think it would be any trouble at all to hook an external amp to my EMX512. Now that I think about it, how do you control the volume on the subs controlled by the external amp? I really don't know the specifics of where you would plug the amp into the mixer or the mixer into the amp. The signal from the audio source goes into any regular channel and then there are a couple of signal only outputs.