both with a Y. I remember doing this in the past.
EDIT: Both L and R of one set of inputs. Use the input selector switch to choose which pair.
https://www.decware.com/newsite/SE84CPLUS%20Owners%20Manual.pdf"To bridge the amp into mono you simply series the output jacks. To do this, take a piece
of wire and connect the left positive speaker jack to the right negative speaker jack. The
two remaining jacks will be used to power the speaker.
Also, it is important to feed a signal into both the left and right input jacks
simultaneously. This can easily be done with a Y-Adapter cable with two male RCA jacks
on one end and a single female RCA jack on the other."
I don't remember if the sound got quieter when unplugging one side or the other.