Ok, I think here's what happened:
- I'm 99% sure the "Parallel" and "Serial" jacks are actually outputs, and they are mutually exclusive options one would use to chain an additional speaker cabinet. Chaining a second cabinet in parallel would lower the total impedance of the system, and chaining one using the serial output would raise the impedance. As impedance matching is important, both options were available.
- Even though they are outputs, they are connected to the input in a certain way, so you could accidentally use them as inputs and get some sound from the speaker.
- Because of how you originally plugged in and how those jacks are probably wired, I suspect any audio that was the same in both the left and right channels (i.e., panned center) would be cancelled out, and audio that was different between the two channels (i.e., panned left or right) would still be heard.
- If you had a delay effect on your rig with the dry sound panned center and delay taps to the sides, then you might have cancelled out the dry sound and only heard the delayed sound, hence the delay in hearing your sound.
Here's how a serial output jack is wired:
Here's how you can have both jacks in one cabinet (imagine the parallel in not being there and the serial out being a switched jack as above):
Note that if the input is wired a certain way, the outputs can be used as inputs instead, depending on how you plug in. Even with that in mind, it's better to consider the "Parallel" and "Serial" jacks to be outputs, in general.