please clarify, you say the amp is pre-owned, i.e. it was/is your existing amp, worked properly, OR, you also just bought it used?
preamp is new, warranty?
cables/speaker wires: still using yours? worked properly previously? solid connections?
as mentioned, you can figure some stuff out by swapping l to r, at preamp, at amp, speaker wires from amp.
leave preamp balance control in the middle, what moves l or r, when, what changed,
my memory sucks I draw diagrams and write stuff down, things get confusing, you end up repeating things .....