Onkyo 7xx series has audio pre-outs, so then you would need just add external amp(s)
like sholling's I have even only 805 avr - it does get hot, but mine is not in enclosed cabinet, but fully open to room air - and I like then it's cool

so does my avr
Putting onkyo avr in enclosed cabinet without extra ventilation = asking for trouble and shorter lifespan.