You can combine EQ Band 1 and 5 for a linear reduction, example:

RME ADI-2 EQ -12 dB reduction with linear frequency response

B1: Gain -12.0 dB  F 1 kHz  Q 0.5 LowShelf
B5: Gain -12.0 dB  F 1 kHz  Q 0.5 HighShelf

The amount of gain needs to be determined subjectively by ear, typically watching the vocal or melodic instruments.
It partly depends on the type of music used.

It will always stay an approximation, as per definition the sound is different.

Now you got me curious.

Typically for minimum phase filters is, a linear frequency-response is accompanied by a linear phase-response.

So, I did a little measurement.
With my EQ / attenuation setting above there is no phase response issue, and voila, as expected, a perfect square wave, indicating all perfectly flat, just pure attenuation (*1):

@MC :
Your proposal works with TotalMixFX (even using two bands), but ADI-2’s EQ’s parameters limit the frequency range of the shelving band #5 down at 200 Hz.

The closest would be using band #1 only, with:
Gain -12 dB F 20 kHz Q 0.5 or 3.8

The result is, that the range above ca. 15 kHz will no be fully attenuated, so neither linear frequency- nor phase-response there.
I spare to post the resulting rippled square wave here.

(*1): A square wave is the fastest way to reveal any irregularities in frequency and phase response.
It covers the whole frequency range.
The slightest deviation from linear in both time or frequency domain becomes immediately visible as deformation of the square.