Speakers seam to go "flat"?

[warr40 10+ year member]

Hello i have some CDT comps on a sundown 100.2

and if i turn the volume up to a decent level, the speakers seam to "dim" or go "flat" not really sure on how to explain it.

What should i start checking?

could it be a voltage drop say when the bass kicks in?

Speaker wire gauge?

Gains?

Please help?

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