The only time an amplifier may change your experience is if your speakers need more support at a low impedance. Add in a challenging phase angle, and you might need that Amp that can deliver 300w into 4 ohms.
For your situation, you added an amp that realistically is only giving you about 3dB more in headroom over your AVR, assuming the gain structure between those units is allowing full power from the Marantz.
Toward what Wayne said above: the Speakers and how you use them is the bigger question!
But in terms of adding an amp and expecting the SQ of a speaker to change? Most amps should not alter the SQ of a signal in any way: they should transmit that signal cleanly and allow your speakers to do their job (which is to take a hopefully clean electrical signal and turn it into sound).
The caveat to this if you are running your system somehow outside of safe parameters, overdriving the amp (causing clipping in the signal chain) for example, then you will hear a changed SQ, for certain.
Please share more info about your system and what you were/are experiencing, please. Also, what is your goal?