Hi and welcome to the forums!
The Pioneer Elites have a good reputation around here. I haven't tried one myself but looked up the specifications and it is rated to drive 4 ohm loads.
It could be that you're simply asking too much of it to play at reference level in your room.
The Marantz that you mentioned has about the same power rating but is not listed as able to drive a 4 ohm load. I would guess that if you are having problems with the Class D amps in the Pioneer Elite you have more chance of problems with the Marantz.
Your Pioneer Elite has pre outs so a better option and what I would recommend is to get an external amp to drive the front channels (or at least L and R). This would leave more receiver power available for the remaining channels.
Decent 2 or 3 channel amps can be obtained used for under $1000. New might be more but some such as Axiom's own ADA1250 or ADA1500 are in my opinion also a good long term buy.
Good luck!
