animuselanvital, this type of attenuator is resistive, and its frequency response isn't limited to the audible range. The specification "20-20,000 Hz" is just to reassure people, since most people don't know Ohm's Law and are concerned only with performance in the audible range.
You evidently take the opposite meaning from those specifications, but that's a misunderstanding: The pad itself is audibly transparent except for the (desired) reduction that it causes in the strength of the signals.
There can be definite differences in quality among attenuators, however. The two series resistors in each "pad" must be very well matched to each other to maintain the balance of the circuit between the microphone and the recorder or preamp. If not, the input of the recorder or preamp will be subject to picking up hum, buzz or RFI from the cable. Fortunately it's not hard to measure that resistance, even without opening the pad's casing.
I've got half a dozen of the older Shure attenuators with fixed 15 dB attenuation; I measured them last year and found that they were remarkably consistent, and that the series resistors in them were extremely well matched. I now have a pair of the newer, switchable type as well, and will measure them, too, as soon as I get a chance, but I would expect them to be as they should be--Shure has always had serious, well-informed audio engineers designing its products.
--best regards