There is more than just ohms to the experience of sound. It is also the efficiency of the speakers that count. I always use a lpad to balance the sound between cabinet and backbox speakers. You put the lpad on the loudest speaker, in this case the cabinet speaker. You quiet this down in this way to come into balance with the backbox speakers.

I have done the same thing recently to my Metallica pin and noticed the same thing, stern has used a cap to restrict the ohms because of the 8 ohm factory speakers, you can bypass this by removing the cap and then you will get a lot more volume.
Only thing is you might notice the back speakers are then louder than the subwoofer, if that is the case you have 2 options, to add an amp to the subwoofer to increase volume or go for the L-pad option for the back speakers to reduce volume.
You can do this quick cap mod in seconds, just twist so the cap is blocked. image.jpg

I'm sorry but this is wrong and needs to be corrected. The cap is there to crossover the sound. It is a 6db high pass crossover. It is not to lower the volume in the backbox. A resistor would be used for that purpose. I do agree with your advice on removing the cap though.

Stern probably used more efficient speakers that were snug to the grill. They also probably wired them in parallel yielding a 4 ohm load at the amp. You got 4 ohm speakers wired in series yielding an 8 ohm load at the amp. The amp can take a 4 ohm load and drives 18 watts at 4 ohms. It's only around 12 watts at 8 ohms. It cannot handle a 2 ohm load. So the 4 ohm speakers were a poor choice if you want optimal backbox volume.

However, I always find the backbox needs to be lowered a lot to get any noticable bass after swapping speakers. I often wire the backbox into a 20 ohm resistor to quiet it even further.

definitely markmon, the cap is there to block lows, and the factory speakers very efficient likely cant handle the lows. possibly try wiring the backbox speakers parallel with a 10-20w 3 ohm resistor in series to them, oughtta be safe enough (5 ohm-ish) but dont know what it'll yield soundwise either. doubt you'll need the cap inline to the new.

Markmon is exactly correct (both accounts) 4ohm speakers in series create an 8 ohm load, thus reducing overall volume of the back box, and a capacitor is used as a crossover.

8 ohm speakers in parallel would have achieved the 4 ohm load goal, though your backbox volume still may have been reduced some due to speaker efficiency.

Greg
www.pinballpro.com