Have you checked if the is visibly torn or crumbling?
Do not replace the original woofers with cheap aftermarket drivers. The original cast-frame Cerwin-Vega woofers are highly specialized. Re-foaming them preserves their original parameters and value. cerwin vega at40 specs fix
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The crossover network uses electrolytic capacitors to route the correct frequencies to the woofer, midrange, and tweeter. Electrolytic capacitors dry out over 30+ years, drifting in value or leaking. This results in dull high frequencies, muddy midrange response, or shifted crossover points. Have you checked if the is visibly torn or crumbling
Let the cleaner evaporate for 10-15 minutes before screwing the terminal plate back into the cabinet. This results in dull high frequencies, muddy midrange
Later AT40s had two pairs of binding posts. Keep the metal jumpers clean or replace them with short pieces of speaker wire for better current transfer.