Maybe it's an era thing (or previous owner mods), but I recall seeing a lot of Altecs in vintage Ampeg BXXs, but it may have varried by model or factory option.
... google is saying Eminence though (in which case, personally, I advise against trying to keep these original unless this is to be sold vs played, but that's a matter of taste).
Either way neither are hard to find, though the Altecs get really pricey. Once you figure out what it is you need, and if you're hard up or in a rush and can't wait for something to come up on ebay, there's a local reconer/speaker seller dude who will have what you need, but wouldn't be worth the shipping otherwise. Lemme know and I'll put you in touch.
Also, what GoT said. If you don't have a multimeter, you can use a 9V battery to test if both speakers are toast (disconnect both, touch battery terminals across speaker terminals for only a brief moment; note if cone moves - yes = good no = bad.... do not hold the 9V batter to the speaker terminals for any length of time, cause if the speaker ain't blown....). Ampeg also was known to use unothodox speakers (such as the 32Ohmers in the SVT cabs, which were custom OEMs by Eminence IIRC) so these aren't necessarily in series. An ohm meter will tell you all you need to know.... also the code stamped on the speaker (if no label) will tell you the manufacturer (post it here).
