Rega Mira Integrated mk1-Gain Settings?

Hi all.
I was given a first generation "Rega Mira" integrated amp, in 'mostly' good working order (front and back face plates have some cracks/damage).
The owner mentioned thinking he had changed the "gain settings" of the amp, but couldn't remember how he did it, or what he had changed it to?
Would anyone know how to do this?
I looked inside the amp, and the only switches are for the "power amp/pre-amp" on/off dip switches.

Possible issue:
I confirmed the amp works but it sounded as though the right channel is a little quieter than the other.
While shifting the amp to better check the speaker inputs, I lifted the right front corner and heard that channel get a little louder/clearer, and then a loud POP.
I instantly let go of the amp, and it all returned to normal sound level and function.
No fuses blown, no smells, speaker is fine (Epos ES14).
Would anyone know what happened?
Did I discover a short or create one?

Many thanks!!!
 

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