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.

Keep in mind that for $30 for a smart power strip and costs of a high level to line level converter, you are no longer near $20.

For about $30 you can have a flipper fidelity quality home made internal sub hooked up. (Under $20 in wpc dcs and wpc 95). I would bet those even sound way better than these cheap 6.5" subs also.

Did you read my big post a few posts up???

Yea. Those converters should address the safety concerns at least.

This is good information. So if the sub is 4 ohms (most are) you do not want to wire both the sub and the RCA inputs in parallel. You would definitely want to unwire the existing sub and use only the powered sub. If you wire a 4 ohm and 11.14 ohm in parallel, you'll end up with a 2.94 ohm to your amp. The pinball amps used generally are only 4 ohm stable. So you definitely do not want to keep your existing speaker wired up and add this without a high level to line converter. Probably best even then is to unwire your existing sub since its not really doing anything anyway.

Does this cheap powered sub have a built in crossover? According to the specs, it doesn't. This means it will play full range and won't really sound good - (Might be ok in a wpc dcs or wpc 95 as those have built in crossovers in the chipset but only at 300hz which is kind of high)

The system will actually sound better if you disconnect the crappy speaker anyway.

I would tap into the wires and add the sub with the high level converter. But there's a good chance the sound sample on those are too crappy to have any lows.