True, but this applies to boosting anywhere, on bass, a pedal or amp. If a particular frequency isn’t there you can’t boost it. However, the frequencies are there, or at least a band of them, they may not just be as prominent as other frequencies, thus the ability to boost (or cut) from anywhere in the chain. At least that’s how I understand it. It’s like trying to process a photograph where certain blacks or whites are blown or blacked out, there’s no info there to manipulate. Fortunately, the likelihood of the total absence of any bandwidth is very unlikely.