Rz3100 is actually a higher model than the rz70. Came out in 2016 and was a current model till 2018, as their flagship avr. Preouts are excellent and should be every bit as good as the rz70.
Why are you wanting an amp? Reason is ask is that avr has a very powerful amp section. Should be very close to any monolith or Emotiva amp. Rated at 140x11 and uses their highest power class d3 module. I had an integra drx7.1 which is a little lower on the food chain than yours. Rz3100 was the sister avr to integra drx1.1 and pioneer elite 901.
rz3100 power should be around these numbers
All number will be .1% distortion full band
170-180 watts by 2 channels@8 ohms
130-140x5@8ohms
120x7@8ohms
if you’re running 7-11 channels off the avr I understand the amp. Remember that onkyo has pre amp modes as well.
Very nice avr!