Totally agree @Neil McRae .
Same with MOST games ever built - the stock speakers have always been notoriously bad to average.
I have not owned a pinball yet that has not been improved by better speaker drivers.
Most stern games are held into the 'eprom' at a fairly good bit rate.... not amazing but fairly good.
Chicago gaming LE/Premiums come out with good speakers, the Classics not so good (but at least now backbox has dual drivers both side unline MMrm did originally), but the sub is still average on the classic.
You do have to watch changing backbox speakers, as you should not really go under 4 ohms! Most dual driver car speakers are 4 ohms. One option is to wire in series which allows more power to go to the sub, or even restrict the speaker line via a pot to the backbox speakers.
Worth a look if you are uprading speakers on the schematic to the amp board to see if the sub goes thru a low pass filter before it hits the power amp chip. There is no point in putting in a crossover on the speaker if it has already been low-passed.
The new @new forest pinball kits are very good value for money, I would get those over Pinball Pro USA kits. Or do it yourelf if you have the knowledge and understand ohmage and speakers
cracked open and 
up IMDN
