The GPA crossovers are just copies of the stock Altec crossovers IIRC.And the crossover for the original 604-8G wasn't very good.You would be better off getting an active crossover (until you can find someone to build them for you).There are lots of folks on the web who will build them for you, some folks even do it for a living.Just ask around on the various audio forums and you should be fine..
Here are some active crossovers..
DCX-2496
Or this one is a good choice as well.
AC-23S
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