I changed the fuse to an Audio Magic Ultimate Premier a while back. The tray slides out about an inch or so and locks without coming all the way out. There are two slots from what I recall where the outer fuse is used in the circuit. I loaded it from the top.
Have you tried the M1 Masterpiece fuse? It is quite a fuse, and I was the one who purchased the used one from USAM several months ago for my Aurender N20.
Yes, it is the same small slow blow 4A fuse used for the Master as well as the MkIIa SE preamp that I had.
Thanks for the good chuckle.
Btw, my Theta will be going in for the CB V upgrade this week. :)