They've been holding the weight just fine for a while now. I think they're rated for 200-250 lbs per shelf.
The only thing I don't like is the way the amp feet sit on the wire. Depending on where things are positioned it may not line up quite right. And the wire will flex some where the feet sit. Not to the point where anything would give and break. That's why I put the plywood, the amps just sat a little better that way than directly on the wire shelf.