My Crown XTi series amp doesn't seem to have space on the amp outputs to connect banana plugs. There's a piece of plastic in binding post holes.
The binding post outputs have a plastic tab covering the banana plug connection because some European power connectors will fit into those plugs (that would be bad of course). If you wish to use banana connectors remove those tabs.
Crown suggest using a small screw like a drywall screw to thread into the tab a turn or two, then pull the tab out with the screw. Of course, do this only while the amp is off and unplugged, and be careful not to damage any other part of the connector.