I'm not sure this is generally true. I've never seen any device not altering the input signal. In the Hi-Fi world, were seeking transparency and linearity is the word, you easily see that every single SS amp has its own sound and personality.I think it's important to be aware that "Tube" power amps will color the sound in some fashion [due to the inherent nature of tubes] and Solid State (SS) amps tend not to.
If you are experiencing sound coloration while using an SS amp, then your tone is being impacted by either the EQ in the preset OR the speakers in the Cab you are using. If the cab is FRFR, then likely it is your preset structure/settings.
What I'd rather say is that, generally, tube amps share a personality on their own and that we, living in a SS world, have probably got accustomed to identify the SS sound with the sound.