Quoted from mbwalker:When you directly connect to the high-level input, you're essentially putting the two woofers in parallel. The cabinet woofer, at least in the Spike 2 Stern's, is 4 ohm. So whatever woofer you connect to it, the impedance ends up being lower.
Quoted from JBtheAVguy:Well...if you connected a passive speaker directly to the pinball machines cabinet speaker in a parallel configuration then yes, you would reduce the impedance by half and damage the pinball machines amplifier. In this case the amplified subwoofer's high level input is being connected to the pinball machines speaker as designed and is not affecting the impedance of the pinball machines amplifier. Think of it this way...if what your saying is absolute then there would be tons of blown pinball machine sound boards. Paper vs. real world.
Just to expand on this, I've sold hundreds of my subwoofer cables (alligator clip style), and I've never had a single customer complain that the setup damaged one of their machines. There have been a few isolated incidents where a certain combination of games/subs caused an unwanted hum or other type of sound, but that's it. This obviously doesn't prove without a shadow of a doubt that there isn't an element of risk, but I think the anecdotal evidence strongly suggests that it's a safe way to add a sub to your game.
