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CS-77A years of manufacture 1972-79 Sealed Acoustic Suspension 8 ohm - real Walnut wood veneer
CS-G203 years of manufacture 1991-92 Ported Bass Reflex - 6 ohm - simulated wood grain vinyl covering (1991-92 was not Pioneer's heyday)
I have the CS-99, CS-99A, and CS-A500, all from that same era 1970-1975. I served in the Navy during that time period, and the speakers of choice in my barracks were the 77A's. Have never owned any of the CS-Gxxx series speakers, but have seen some in thrift stores and yard sales - the build quality is crap compared to the older CS-77A.
Without hesitation my choice would be the 77A. and they match up with the vintage of your receiver - SX-727 produced 1972-74
Does your 77A have the correct 'FB' cones?
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You can get a set of foams for the 77A tweeters for under $20. I wouldn't spend $38 on the G203.