Just split one of the sub pre-outs if needed, doesn't need to be daisy chain type connection. The 8500 caps out at 13.1 ch of processing IIRC....but the new
A1H has not only the 15ch of processing but four sub pre-outs separately configurable for delay/level. The best to keep using your old avr is use it as a power amp, there's no add-on unit for the additional processing needed....