It's complicated, but in simple terms if you are not using a pinsound board, best to keep it in series. (Giving a 16 ohm load). for the sam board system this is easier to drive than wiring in parallel, as this results in a 4 ohm load. A lower impedance speaker requires greater current from the amplifier to provide the same voltage at the output. That is typical, simple ohms law. (For example 1v/8ohms = 0.125amps. 1v/2ohms= 0.5amps. ) But other factors are involved , for example the frequency range, and sensitivity of the flipper fidelity speakers. Not knowing these specifications, leads to a suggestion. Try it both ways and consider the following..
1). What is the difference in the sound quality and volume levels.
2). Check for distortion (clipping). At higher volumes, and the power I.c on the sound board getting too hot. (It will always run fairly hot). Experimenting for a short period of time at sensible volume levels will cause no damage.
If either of the things occurs in point two turn the machine off......
Remember the stern stock speakers are complete garbage. They have a narrow frequency range, high sensitivity and high impedance so that when wired in series they sound loud with minimum strain on the sam amp. Circuit........