Quoted from vid1900:

Things to remember:
1. Those old crappy speakers were what the sound was tuned and equalized to when the game was made. People often install car stereo speakers and the sound is then very harsh and bright.
2. Efficiency is EVERYTHING when it comes to driving speakers with a little 4w amp. When people say their pin is too quiet after installing new speakers, that is often because the new ones are less efficient.
A speaker that is 3db more efficient, will be TWICE as loud as one that is 3db less efficient.
Speakers will give their efficiency usually with a rating like " 92db 1w/1m ". That means that the speaker will deliver 92db when driven by 1 watt of power, measured at a distance of 1 meter.
The higher the db number, the more efficient the speaker is.
Never buy a speaker that only says "85db" without giving you the wattage and distance. If they can't give you all the needed information, you can be sure it's a super crappy speaker.
3. The Ohms ( little horseshoe symbol) is important on a speaker because a well designed amp will put out 2x the watts when you cut the Ohms in half. So a good amp that puts out 10 watts into 8Ω, will put out 20 watts into 4Ω.
When people say their new speakers are too quiet, even though they are very efficient, they often bought 8Ω speakers to replace the original 4Ω; thus loosing half the watts in the process.
4. Your largest gain volume vs. dollar on a pinball machine is getting an external sub. You can hook up a dozen games to a single subwoofer with a small line mixer.