Broken banana plug and stuck in amplifier

not rubbing salt in the wound and hope you get it out soon.  It shouldn't be hard to get out with proper tools.  I haven't made any recommendations because I don't know your skill level and tool inventory.

I want to point out that these inferior multipiece banana plags significantly affect sound (negatively) and eventually fail.  This is what drives people to lugs.  I like BFA style banana plugs.  One piece and great electrical connection  (again, avoid the multipiece designs).

Jerry

Second the idea of using a screw.  Insert and hand tight (just snug), want to embed the threads in the broken banana plug, but not get expansion that tightens everything inside, then use some pliers to gently pull on the screw.

One other possibly is a set of dental tools which can be found at at most pharmacies.Weird shaped picks might be able to hook it right out.

I'd unscrew the binding post nuts as far as possible, and try to pry/push it out through the hole in the side with a small punch or a big needle or something.  If you get it up a little bit, you should be able to screw the nut back down and get at it with some pliers.

are you trying to save the plug or the amp?