I don't think they suck but I like the DIY approach.
For the most part, you need to do more than just throw speakers at a pin in order to improve it's sound quality. If you want a DIY solution, you can also learn a lot in the process.
First problem I see with your speakers is that you installed two way speakers where woofers once were. Putting a two way where a woofer was is going to sound like ass.
My advice it to use a woofer where a woofer was and use a mid range where a mid range was, ect.
If you want to just throw anything into a game, at least get a cheap ass $30 amp that you can plug in to the service outlet on your game, add a small surge protector with a switch, then take the sound source from the game, run it into the amp, and then run your new speaker(s) from that amp. Then it will sound a lot better than just throwing two ways in place of the original $5 woofers that Data East/Stern used!
I have used this small beauty no less than 3 times and it is the best $ you can spend for what you need to get from factory 20 year old pinball sounds:
It's not just about the speakers!
Once you feel confident you have an amp and speakers that sound good, you could then fine tune the set up it with some homemade low pass and high pass filters using spare caps you have lying around...
You can improve the sound on ANY game but most people don't want to learn the lessons required to get it sounding amazing and just op for the packaged solutions that the companies mentioned above make. Those setups can work great but I have discovered that games with true stereo sound (like Data Easts) can be more difficult to get sounding great.