Quoted from markmon:

You guys should know that line level input is not intended for speaker wire inputs. Those are found on sub amps that say "high level input". High level inputs are designed to show almost no added resistance to your amp. This is very important as it prevents over heating and blowing your amp. Using speaker out to line level input may produce an undefined load to your amp causing it to heat up or blow. In this case, the amp is your av board amp in your backbox. On a 20-30 year old game, you definitely do not want to stress those components even further. And saying "it seems to work ok for me" is no response to this as it may work for a while while stressing the board in your pin to failure. Before attaching the speaker wires to this thing, someone needs to measure impedance on the RCA jack input at the very least.
The way to properly wire this is to intercept the line level input on the av board before its sent to the internal amp.
Further, wiring line level input up to high level input will add a lot of distortion as the sub input is expected to be a set "line level" volume.
Further, not having auto sensing means this thing will be constantly powered and generating heat 24 hours a day even when your machine is off, assuming you use the service port. The OP's instructions for modifying the port must be used. Or cut the power cord and wire directly to the white / black wires near the coin door for the bill validator. These are switched 110v. Remember if you mod your service port, you can't plug in a soldering iron when machine is off to do repairs.
Therefore, I would not use this in a pinball machine.