every arcade unit i've ever heard with computer speakers sound like poo in my opinion.
I would suggest a trip to the local best buy/electronics store and pick up some coaxial/triaxial car audio speakers and a small amp. If you want good bass, I wouldn't go with anything smaller than 6", 6x9"s being prefered.
I have some pioneer triaxual 6x9"s with a sony 60w (per channel) amp in my cocktail and have no need for a sub. You realy havent played asteroids until the explosions rattle everything in the room, and this system does it.
As far as bucking magnets go, make a trip to the local junk yard and snatch a pair of speakers out of something. GM vehicles are usualy the best donors, and condition doesn't matter as long as the magnets are good. If the speaker is obviously bad (i.e. rip the cone off) they'll probably just let you have them.
Then put each speaker in a vice resting on the lip of the magnet itself. Take a punch and a hammer, and give a couple of wacks to the center of the pole. back end should come off and you'll have a nice bucking magnet.
But, thats just my DIY approach to it.
-Harry