I have an old, ancient, Grampian Spring Reverb here. BBC design. It's output can drive a loudspeaker.
I would avoid that Mono switch. I never tested this but I never liked what happens when it is engaged. I think it just connects outputs together.
Also, as I don't power my unit, no LEDS..... The Mono switch got knocked when dusting or hoovering once. I started to have mysterious issues, until I eventually found the displaced switch.


DD