Mounting CNAMPX-16x60 by rack ears?

It should be fine to mount it with just the front ears. Most amps mount that way. If the unit will mount square without deflection or distortion of the front plate/rack rails then it should be OK.

The 4 post thing is common for deep equipment, like 1U servers, and equipment with slides for service reasons.

Do yourself (and your back) a favor and keep it mounted at the bottom. Just make sure there's enough space above it to allow for proper convection cooling. You don't want to have that amp cook whatever's mounted above it. That much weight on a two post rack is asking for trouble if it's mounted higher up. What sort of rack posts are you using? How is the rack secured? I'd also worry about twisting issues and plan for securing against the risk.

If you're not concerned about being flush-mounted there are offset shelves for racks. That way you'd be able to shift the weight forward such that it's better balanced.

For something that heavy I'd be much more likely to spec a 4-post rack.

-Bill