[caprireds]

If an external Headphone amp has

Frequency response

20-20kHz±0.6dB

does it mean that it won't get any signal/frequencies beyond that?

for example if i connect the sennheiser HD 598SE that has Frequency Response 12 – 38,500 Hz , the amp won't feed them with their full capacity of frequencies?

 

[MohawkUS]

20-20khz is a standard rating as its the standard range of human hearing. You also may have companies noting wider range for marketing purposes if the amp can do it with minimal roll off.

The rating given does not tell you the severity of roll-off outside the range given so it cannot be answered what the amp is capable of reproducing. All things being equal you're likely using a digital source with a filter which removes all content above about 22khz. The behavior of the amp is going to be less important than that of your DAC at the extremes of the audible range.