This problem could be cause: BAD AMPLIFIER RESPONSE, CABLE PROBLEM, PASSIVE PROBLEM.
Your amplifier should be equiped with response control, where you need to tune the response of the amplifier.
One thing you can do, is sweep the INPUT of the amp. you are working on And if it meets the maximum frequency of your system, then do the output of the same amp. this should give you a good idea of the problem.
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