Broken banana plug and stuck in amplifier

blowtorch and arc welder 

Or get a pick set, quite handy to have around. That is if the suggestion @jjss49 recommended fails to work out.  

https://www.jbtools.com/ullman-devices-9-75-in-hook-and-pick-set-with-cushion-grip-8-piece/?wi=off&gclid=CjwKCAjwsvujBhAXEiwA_UXnAO--MDSoM2cUzqGrPd0Y4iBMrSez4NAmhEn2CUwYtrbiL7IM6mn7FRoCdbUQAvD_BwE

A wood screw should be able to engage the inside of the broken banana plug, then use pliers to remove it.

That is a tough one.  Every situation is different and depending on how/why the banana plug broke off, it may be slightly wedged in the socket of the binding post. 

It seems the easiest solution would be to unscrew the knob on the binding post, if that is possible.  Next would be to use a small tool to loosen and free the broken banana plug.  Even if you cannot grab the broken piece, try using a small tool to turn or move it a little, and then try turning the amplifier on its back so the opening in the binding post faces down and maybe gravity will help it fall out. 

The plastic posts on the E280 are mounted on a small circuit board so if you cannot pull out the broken banana plug and cannot unscrew the knob to the binding post, then the worst case would be to replace the binding post assembly, which appears to be attached by a quick connector and a single ground wire as shown here.

take the cover off and push it out with something