The header logo at the top of this page also reads "A discussion forum by Audiogon".
Asking here about removing tubes from your SLI-80 to use it in a way it was not designed to be used, most would simply advise to contact the manufacturer with your serial number. There are cautions in the owners manual about removing the bottom plate, with a logical safety reason for this.
Some designs use filter caps with a sufficient voltage rating to handle no tubes plugged in, then checking B+ voltages, plates, screens. Who knows what’s installed or been changed in your amp since new. Various caps were used in SLI-80s over the years. Since you came to the Tech Talk forum looking for technical information, do you know what’s installed in your amp now? Do you know how to check values? If not, back to logical step#1.