Ground Zero GZHA4120X issue

Hi, i have this Ground Zero GZHA4120X, that i never used, i was testing it comparing to other amplifiers (jbl, edge, mac audio, etc etc normal mass market amp) and i noticed that the highs on this GZ are much softer and warmer, almost like it has no highs, to my ears not being that clear on voice and highs...all other amps that i tested had more brighter highs.
Is this a general characteristic of the GZ amplifiers or it might be something wrong with my amp?

Never heared another GZ's amps so don't know if this is how they sound.

This being nice and shiny (2 chrome plates) was thinking to buy another one of 4channels and do an open rack in the trunk, and now i don't know what to do.

Thanks.
 
on the actual amp no, i've just replaced multiple amps on my current setup with the speakers passive crossover, so there is no passive crossover issue
so it's 10 Hz up to 38 kHz
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yes it is set to flat, all my aps have flat xo 😉, i will try to find some other GZ from same series to compare, it's either the sound..or some electronic crossover failure,. I've heared for example some other amps that have a mellow and softer highs but...not quite like this one
 
i don't have a dedicate multimeter with true rms function, just normal one
i'm building a kit scope right now, i think it's pretty ok and can measure sine waves pretty ok, i bought the kit to measure 1khz amp clipping for gain and HU clipping
how could it help me determine my issue?
 
You'd run a sweep or a set of fixed frequency sine waves and see if the output of the amp is at the same level at all frequencies.

If you're building something like the DSO138, you need to confirm that it can display all of the frequencies up to 20kHz, flat. Some of the cheaper scopes are not very reliable,