Think about it for a minute.
Assuming you ran ARC, that means the loudest any mains speaker can ever be asked to play given the volume settings at which you listen is 100dB at the listening position. (IIRC, -10 is actually reference level on the Anthem boxes.) Let's be conservative in balancing listening distance with output from reflections, and say that means 103dB/1m. (You can be more precise by downloading an SPL meter app for your phone - JL Audio has a free one for iPhones - playing a test tone at a comfortable volume, taking a measurement ~1m away, and taking a measurement at the listening position to see how much output actually fades in your room.)
Your mains are rated 88dB/W/m. That means you need about 15dBW of power to get 103dB out of your mains. The MRX300 clipped at ~70W with 7 channels driven in HT Mag's tests, and is rated at 60W/ch. Peak output will be higher than that. Seventy watts is ~18.5dBW, and 60W is ~18dBW. So even assuming 3dB of power compression, the MRX alone is more than adequate to drive your speakers to your required levels.
So the bottom line is, all an extra amp will buy you is marginally higher power bills.