Likely that you get to the crossovers by removing the woofer and any filling that's inside the speaker. If there is any sort of tweeter level control, it's likely that corrosion in that control is the problem. This was/is a common problem on AR 2a/3a speakers. It's unlikely that the crossover itself is causing the loss of highs.
Once you get to the crossover, you can see if any caps are bulging or find high quality one's on the internet...but unlikely that's your problem.
Once you get to the crossover, you can see if any caps are bulging or find high quality one's on the internet...but unlikely that's your problem.