removing mark3 amp chassis from combo

If I remember correctly, you have to unplug the fan too.

The amp chassis will come out easily by just lifting slightly, while gently pulling and "walking" the chassis back toward the back of the cabinet.

Good luck.

Why not: Unplug and remove the speaker - place the amp on a table, upside down on the handle - place the speaker behind the amp on the table - all the tubes are now easily accessible through that hole where the speaker used to be.
Less chance of electrocuting yourself by touching stuff inside the exposed chassis, even if the amp is unplugged. You did know this, didn't you?
If you're going to remove the chassis, you should remove all the tubes first - make note of which socket they came from.