Quoted from PinballTilt:

Does anyone know the sensitivity of factory stern speakers? Specifically this is a jp premium, so the newer ones. I'm trying to understand all of this speaker information and one thing I've picked up is you need an efficient speaker for pinball machines. If you don't have an efficient speaker and you only replace the cab speakers with inefficient speakers, the balance between cab and backbox will be messed up. The only way around that is to add resistors (or get new speakers), but I have no idea how many ohms of resistance I would need to add because I don't know the sensitivity of stern speakers. According to Amazon the speakers I bought have a sensitivity of 82 dB

Quoted from Markharris2000:

It's perhaps better to discuss speakers for pinball in your own words rather than trying to compare speaker specs, since most folks won't really care as much about rated efficiency or sensitivity. I would suggest that what they care about is :
1) Is the sound quality of the game good? Does the audio sound crisp as it should be, and can they feel the lows. Do voices sound natural? Are sound effects believeable?
2) Is the volume enough for the space you are playing the machine? In a small room, you typically need less volume, but in a larger room you could use more.
Best to describe what you are hearing (or not hearing) and then decide what to do and see what others are doing...