The amp schematic shows that the headphones jack is actually in parallel with the speaker output jack connections. So that would make me think that anything you hear out of the speaker then would be the same with the headphones. Start by removing the speaker from the amp and then check with a voltmeter set to read DC voltage to check for DC offset. If there is DC voltage at the output it can burn up the speaker so you want to see only -/+50mv or less typically speaking. A speaker constantly pushing outwards will indicate DC voltage is present on the speaker. The power amp transistors might actually be okay and still we might see a small 7-10vdc on the output if some other small transistor shorted out. Start there and let us know what you find. Another thing to read is this post here... The effects return jack could be dirty and plugging a regular guitar patch cable from send to return jack will help determine if that is the problem.
Edit: originally wrote -/+50v at speaker output but meant -/+50mv. Oops