Basic help needed to open a case

[JMFahey]

Think "if I do nothing to something it will stay the same" and you´ll get it.
Maybe

 

[camrector]

There is a screw in the dead center of the rear.
Take that off and the long screws on the bottom and it slides right off. Easy Peezy.

 

[JMFahey]

JMFahey said:
All ruler flat vanishing distortion fast devices sound the same, by definition.

Whoa, that's a funny "definition"!
(But it fits how this thread is full of "I know stuff and I am right, but you don't know , by definition.").

Prove me wrong.

 

[ulogon]

There is a screw in the dead center of the rear.
Take that off and the long screws on the bottom and it slides right off.

It seems that the OP finally managed to remove the cover and also returned the preamp.
https://www.audiosciencereview.com/...f-gustard-p26-preamplifier.10492/post-1329768

However, it might be of interest to some owner that here too is the complete procedure explained in an even more detailed way.
https://www.audiosciencereview.com/...op-cover-removal-procedure.39166/post-1379396