I have a MD-100HDFX Marshall Stack - it has a headphone jack, but when I plug my headphones in, the sound still comes out externally and not through the headphones - is there a specific setting I need to change or a switch that I'm not seeing so I can play and listen just through my headphones ? The factory manual doesn't explain - but the input specifically says "headphones".

The model you have is probably splitting the channel and distributing it through the headphones and through the amp. This is so you can record the guitar through 2 different sources at once, the cabinet, and as well as through a cable in direct input. There should be some kind of external output jack that kills off the power in the cabinet, if not then you might have to get a different head. I haven't seen many marshall heads that actually would go straight through headphones.

Try turning the master volume down to zero.

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Miami

This may sound harsh, but up-grade to a JVM 410H with two 1960A/B cabs underneath and say **** the headphone jack!!!

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THE TIME HAS COME FOR THE REAL ROCK AND ROLL MUSIC TO RESURFACE AND TAKE OVER!!! REVOLUTION IS HERE;THE RESURGENCE HAS STARTED!!! REAL ROCK AND ROLL WILL RULE AGAIN!!!