Quoted from cody_chunn:If you're going to install two 4 ohm speakers in place of one 8 ohm, hook them up in series so the load impedance remains the same. That would be sound board + out to speaker 1 + lug; out of speaker 1 on - lug; in to speaker 2 on + lug; out of speaker 2 on - lug; then back to sound board - return.
This is a good way to expand the special volume of sound while maintaining the 8 ohm load it was designed for.
I just did this with some Kickers. The end result is a big improvement in my opinion. There is a wiring diagram that was very helpful in this thread along with links to speakers that are a direct fit(Thanks to Nosro).
https://pinside.com/pinball/forum/topic/anyone-update-the-speakers-on-their-mmr/page/2#post-3335920
