The Marshall AVT275 is a hybrid amp. Hybrid amps have a tube in the preamp stage that warms up the signal before sending it to the digital output stage. The AVT275 uses a 12AX7 preamp tube. In addition to running the signal through the tube itself, the AVT275 also runs several of its on-board effects through the preamp tube. Since tubes have limited lifespans, the effects that run through the tube will start to fade as the tube ages.
Unscrew the four screws on top of the amp to remove the preamp. Unscrew all of the screws in the back of the cabinet as well to gain access to the speaker.
Check the wire leads attached to the speaker. If they are loose or disconnected, solder them back into place.
Check the preamp tube for black, white or silver spots. A broken or burnt-out tube will look similar to a burnt-out light bulb.
Unscrew the fuses on the back of the preamp, near the power cord. If there are burn marks, the tube is either dead or will be soon.
Check the circuit board for any burn or black marks. These indicate that one of the components has shorted out or burnt out.
Don't worry about scratchy knobs. It's normal to hear a scratching sound when turning a volume or effect knob, unless it's the "Presence" knob. Completely burnt-out tubes are rare. Blown fuses are more common, although they may indicate that a tube is ailing.
Always unplug the amplifier from its power source before checking or repairing it. The preamp tube can hold electricity even after the amp is off, so unplug the amplifier at least two hours before beginning work. Wear gloves while working. If your amp is under warranty, remember that you void the warranty by taking it apart. Be careful when unscrewing the preamp, as the screws are the only thing holding it up. Keep your hand under it to keep it from falling.